A conventicle originally signified no more than an assembly and was frequently used by ancient writers for a church. At a semantic level conventicle is only a good Latinized synonym of the Greek word for church, and points to Jesus' promise in Matthew 18:20, "Where two or three are met together in my name."
Over time, it came to be explicitly applied to meetings of religious associations, particularly private and secret gatherings for worship. Later it became a term of deprecation or reproach, implying that those who used it opposed the ruling ecclesiastical authorities; for example, it was applied to a plot of mutinous monks in a convent or monastery.
Ultimately it came to mean religious meetings of dissenters from an established church, held in places that were not recognized as specially intended for public worship or the exercise of religious functions. It implied a condition of affairs in which the state made a distinction between the forms of religion whose practice and propagation were authorized by statute, and those that were expressly prohibited. This usage has received legal sanction in Britain.
Jesus' disciples as conventiclers
By the accepted usage of the word, Church historians properly assert that Christianity rose ecclesiastically from a conventicle. Such was the meeting in the Upper Room of the first disciples of Christ after the Ascension (Acts 1:13).
This gathering was the type that soon began to take place for prayer, mutual edification, and memorial observances in private houses such as that of Mary, the mother of John (Acts 12:12). Within a short time, they drew upon themselves the suspicions of the Jewish ecclesiastical authorities, which branded the new faith as impermissibly heretical, and instituted a persecution directed at the harrying and suppression of these conventicles, one of their most zealous agents being he who became the Apostle Paul. 
Conventicles in the early Roman Empire
When Christianity became a world religion and spread in all directions throughout the Roman Empire, it was at first tolerated and enjoyed government protection, along with many other cults in vogue. Religions had to receive a license from the state, which was jealous, to secure itself against the danger of conspiracies maturing under the guise of religious confraternities. Largely through the influence of political considerations, Christianity soon became suspect, and a religio illicita. Its meetings thus became strictly conventicles.
Historians have used the term to characterize such house meetings as mentioned in Col 4:15. In the succeeding century, the catacombs were the scene of Christian conventicles. 
Conventicles in the later Roman Empire
With the establishment of Christianity by Constantine as the state religion, all its meetings were legitimized, and the term odium could no longer be rightly applied. In the 4th and 5th centuries, the description again became applicable to the meetings of such Christian nonconformists as the Montanists and the Donatists, which were prohibited by the state under penalty of prescription and death. This policy was rigorously encouraged by the leaders of the churches enjoying state recognition and support. The 6th canon of the Synod of Gangra pronounces against conventicles.
Conventicles in pre-Reformation Europe
In England, the word was early applied to the meetings of the followers of Wyclif, who, recognizing the incompetence and neglect of the regular clergy, sent out peripatetic preachers to meet the spiritual needs of the people. Both the practice and the word were carried by the Lollards (as the most determined supporters of Wyclif were called) to Scotland, where they did much to initiate or strengthen the movement of revolt from the ecclesiastical domination of Rome.
England under the Tudors
It was not until after the Reformation that 'conventicle' became a term with a legal connotation, according to which it was descriptive of the meeting place or assemblage for worship or consultation of those who departed from the Established Church of England. Queen Elizabeth, in her contest with Puritanism, strenuously asserted the royal supremacy in matters religious and ecclesiastical and insisted upon the rigorous application of the Act of Uniformity, which demanded that all subjects of the Realm must conform to the usages and tenets of the Church established by law. Clerical nonconformity was punished by deposition.
As a result of the following inquisition, so many ministers were deprived of their lives that their places either could not be filled at all or were filled by incompetent and unpopular substitutes. Many people refused to accept the ministrations of these substitutes and gathered together for worship in private houses or other suitable places. These conventicles were, under that name, expressly declared illegal. The 11th Article of the Book of Canons (drawn up in 1603) censures "the maintainers of conventicles"; the 12th, "the maintainers of constitutions made in conventicles"; and the 73rd runs thus:
"Forasmuch as all conventicles and secret meetings of priests and ministers have ever been justly accounted very hateful to the state of the Church wherein they live, we do ordain that no priests or ministers of the Word of God, nor any other persons, shall meet together in any private house or elsewhere to consult upon any matter or course to be taken by them, or upon their motion or direction by any other, which may any way tend to the impeaching or depraving of the doctrine of the Church of England, or the Book of Common Prayer, or any part of the government or discipline now established in the Church of England, under pain of excommunication ipso facto."
Under these enactments, the adherents of Anabaptism, which had been propagated in England by refugees from the Continent, were ordered to leave the Kingdom. Even during the subsequent reign of Puritanism, the meetings of this particular body were regarded and treated in the same fashion by the Protector Cromwell, who was incensed by their aggressive fanaticism. There was a breathing space of toleration and freedom for other persecuted sects, with only one or two exceptions.
The Religion Act 1592, stated to last for just one parliament, called for imprisonment without bail of those over the age of sixteen who failed to attend church, who persuaded others to do the same, who denied Her Majesty's authority in ecclesiastical matters, and who attended unlawful religious conventicles.
The Conventicle Act 1664 forbade conventicles of five or more people, other than an immediate family, from meeting in religious assemblies outside the auspices of the Church of England. This law was part of the Clarendon Code, named for Edward Hyde, 1st Earl of Clarendon, which aimed to discourage nonconformism and to strengthen the position of the Established Church.
The Conventicles Act 1670 imposed a fine of five shillings for the first offense and ten shillings for a second offense on any person who attended a conventicle (any religious assembly other than the Church of England). Any preacher or person who allowed his house to be used as a meeting house for such an assembly could be fined 20 and 40 shillings for a second offense.
England under the Stuarts
After the Restoration of the Stuart dynasty, established Episcopacy once more became intolerant under the aegis of Charles II. An Act of Uniformity was promulgated in 1662, which ordained the expulsion from his charge of any clergyman who refused to subscribe to everything contained in the Book of Common Prayer and to the doctrine of the King's supremacy in matters ecclesiastical, and held by the Solemn League and Covenant of 1643, prohibiting such from exercising his religious functions in private houses. In one day, two thousand clergymen were ejected for failing to comply with these tests. This enactment was reinforced in 1664 by a statute called 'the Conventicle Act,' which rendered illegal any gathering in a private house for religious worship attended by a number exceeding five the regular members of the household under penalty of fine, imprisonment, or transportation. A second version of this Act deprived these outed ministers of the right of trial by jury. It empowered any justice of the peace to convict them on the oath of a single informer, who was to be rewarded with a third of all fines levied. Large numbers of nonconformists were put in jail. Pepys, in his diary of August 7, 1664, observes: 'I saw several poor creatures carried by, by constables, for being at conventicles ... I would to God they would conform.' He refers to Quakers, amongst the worst sufferers during the persecution consequent on the passing of the Acts. Bishop Burnet, in his History of his own Time, admiringly describes how they resolutely declined to obey the law and openly and fearlessly continued their prohibited meetings. They would hold them in the street before their meeting houses closed doors when these were shut by order. The children, who might not be arrested because of their youth, would also hold conventicles in the street without their parents in jail, suffering patiently the jeers and cuffs of magistrates and unsympathetic onlookers. 
Scotland under the Stuarts
There were various answers given in Scotland as to how to react to the persecution of Presbyterian Christianity under James VII and Charles I. Scottish ministers did not regard complete separation from the Church of Scotland as an option.
Identical measures were taken in Scotland under Charles II as had been taken in England, only now it was to secure the suppression of Presbyterianism in Scotland, where it had been the popular and dominant form of religion since the Reformation. From 1662 to 1678, various Acts were passed by the Privy Council and the Court of High Commission, prohibiting conventicles and imposing penalties of increasing severity upon those who attended them, masters being made responsible for their servants, landlords for their tenants, magistrates for the citizens of the burghs over which they presided. It was forbidden to supply denounced persons with meat or drink or to harbor or have intercourse with them. These measures proved unavailing to effect their purpose, and it was ultimately enacted that attendance should incur the death penalty. Those in command of the military, and even the common soldiers themselves, were given authority to inflict it immediately on the spot of capture without the formality of a legal trial. In numerous instances, this authority was used without scruple or mercy by such as Claverhouse. This policy proved, however, quite abortive. The bulk of the religious population in the south and southwest districts continued to attend the conventicles arranged and conducted by the outed ministers. Where the congregation was too large for a suitable private house, the resort had barns, granaries, or commodious buildings. The number of those who flocked to these illegal gatherings frequently amounted to thousands. The result was the institution of field conventicles' — meetings held, sometimes under cover of night, in the open air, on moors or hills, in glens and ravines, or wherever safety and suitability could be combined.
These frequently lasted for hours, the preaching taking up much of the time. At such conventicles, the ordinances of the Church according to Presbyterianism were faithfully observed. Baptism was administered, and Communion was dispensed, often to hundreds together, and even thousands, the rite taking days to celebrate, several ministers officiating in turn. When repressive measures became more severe, and attendance at these gatherings was enacted to be a capital offense, the men came armed with such rude weapons as were obtainable — scythes, flails, etc.
A Presbyterian theology covering, amongst other topics, conventicles and even armed resistance to tyranny was given in Alexander Shields' work A Hind let Loose. It is (says MacPherson) in the second half of the book that Shields’ power as a thinker is manifested. Under seven heads, he discusses the day's fundamental social, political, and ecclesiastical questions. These heads are concerning (i) hearing of curates, (ii) owning of tyrants’ authority, (iii) unlawful imposed oaths, (iv) field meetings, (v) defensive arms vindicated, (vi) the extraordinary execution of judgment by private persons, and (vii) refusing to pay wicked taxations vindicated. The last-named section was added, Shields tells us, as an afterthought.
Sentinels were posted at lookout points; for the royalist soldiery, aided by spies and informers, often succeeded in surprising these meetings. It was the attack upon such a conventicle that precipitated the Battle of Drumclog, 11 June 1679, which issued in the only victory gained by the Covenanters (as the upholders of Presbyterianism were called), and the only defeat sustained by Claverhouse (known in song as 'Bonnie Dundee'), the most zealous and efficient of the military persecutors. During the years of persecution culminating in the 'Killing Times,' it is calculated that some 18,000 people suffered in one way or another for attending these conventicles.
The Rescissory Act 1661 repealed all laws made since 1633, effectively ejecting 400 Ministers from their livings, restoring patronage in the appointment of Ministers to congregations, and allowing the King to proclaim the restoration of Bishops to the Church of Scotland. The Abjuration Act of 1662 .. formally rejected the National Covenant of 1638 and the Solemn League and Covenant of 1643. These were declared to be against the fundamental laws of the Kingdom. The Act required all public office persons to take an oath of abjuration not to take arms against the king and reject the Covenants. This excluded most Presbyterians from holding official positions of trust.
The resulting disappointment with Charles II's religious policy became civil unrest. It erupted in violence during the early summer of 1679 with the assassination of Archbishop Sharp, Drumclog, and the Battle of Bothwell Bridge. The Sanquhar Declaration of 1680 effectively declared the people could not accept the authority of a King who would not recognize their religion nor commit to his previous oaths. In February 1685, the King died and was succeeded by his Roman Catholic brother, the Duke of York, as King James VII.
James was eventually deposed in England in favor of his nephew, the Calvinist Stadtholder of several provinces of the Netherlands, William III of Orange and his wife, James' Protestant daughter Mary. In Scotland, a Convention of the Estates was called in Edinburgh. At this convention, it was decided after considerable deliberation that England, having been conquered by William of Orange and his troops with little or no resistance, Scotland would support William and Mary's claim to the throne of Scotland. However, in the ensuing rebellion against the Williamite coup, some of James' loyal followers – the original "Jacobites," among whose ranks were many Highlanders – inflicted a heavy defeat on the new Government's forces at Killiecrankie. The redcoats at this battle were no locally raised militia. Still, they were, in fact, the renowned Scots Brigade – a famous unit of Scottish professional soldiers in Dutch service, some of whom had come over to Britain with William. Thus in a bizarre twist of fate, it fell to a small band of men self-consciously called the Cameronian Guard after the rebel followers of the martyred Covenanter preacher Richard Cameron, to defend the new Government in a small but significant battle fought in the streets of Dunkeld against the recently victorious Jacobites. Thus former rebels fought to uphold the once-again ascendant Calvinist Protestant order in defense of the Covenant against the defenders of the old Episcopalian and Roman Catholic establishment. The Cameronians managed to hold out long enough for the Government to bring reinforcements and for the Jacobite advance to falter. The tables were now turned, and once the rebellion was defeated, the Cameronians, heirs to the victims of government-mandated "pacification" at the hands of units like the Scots Greys, were used to police the Highlands and restore order.
Ejected preachers such as John Blackadder conducted religious ceremonies at conventicles. Many of the covenanting prisoners on the Bass Rock had been charged with attending conventicles.
Scotland after the Revolution
After the Revolution of 1688 and the accession of William of Orange to the British throne, an Act of Toleration was passed, relating to England, which exempted from the penalties of the laws against conventicles those who took the oath of allegiance and subscribed to the doctrinal sections of the Thirty-nine Articles. Meeting houses were required to be registered and then came under the protection of the law. In Scotland, all the repressive Acts were abrogated; Presbyterianism was somewhat restored by the State to its ecclesiastical supremacy, although there were some Cameronian Dissenters among others who did not like the terms of the restoration. There was more tolerance in Scotland after the revolution, even before the legislation regarding the Establishment of the Church of Scotland at the Act of Union.
During the relentless and prolonged attempt of Philip II of Spain in the Netherlands to compel conformity to the Roman Catholic Church, the Protestant party headed by Les Gueux ('The Beggars') were forbidden free exercise of their worship. Immediately field preachings were organized all over the country, of the same character as those in Scotland — conducted by the excommunicated ministers and surrounded by armed guards and sentinels. 
The same scenes were enacted in the southern districts of France during the struggle of the HuguenotCamisards (les Enfants de Dieu, as they called themselves) to assert religious freedom against the suppressive measures of Louis XIV, inspired by Cardinal Richelieu's vision of a unified France, spurred by the incitements of Madame de Maintenon (herself once a Huguenot), and encouraged by the eloquence of the great preacher Jacques-Bénigne Bossuet. Their field conventicles were called desert preachings — the name 'desert' is borrowed from the Bible as descriptive of the remote places in wild mountain regions where the meetings were commonly held. Antoine Court, for example, led the church while living in dens and holes in the ground. Paul Rabaut lived a similar lifestyle living as roughly as Alexander Peden in Scotland. A peculiarity of these Camisard gatherings was the large part played by the 'prophets' — men and women, and occasionally children, generally quite uneducated and often normally of small capacity for speech or thought — who spoke or were accepted as speaking under the direct inspiration of the Holy Spirit, after the manner of the prophets in the primitive Church (Peyrat, N. Histoire des pasteurs du désert, Paris, 1842; C. Tylor, The Camisards, London, 1893).
In the history of German Protestantism, the conventicle played a part in Pietism. The collegia pietatis, established by Philipp Spener and his followers, provoked the opposition of the strictly orthodox Lutherans, and considerable disturbance was the result, as at Frankfurt, where the police interfered. All sorts of scandal were rife about these conventicles, and the over-enthusiastic manner in which some were conducted lent color to the charges. In Württemberg, a wise middle course was adopted. Those conventicles in which the great principles of Lutheranism were respected received a legal sanction, while the more radical assemblages were banned.
Philipp Jakob Spener called for such associations in his Pia Desideria, and they were the foundation of the German Evangelical Lutheran Pietist movement. Due to concern over possibly mixed-gender meetings, sexual impropriety, and subversive sectarianism, conventicles were condemned first by mainstream Lutheranism and then by the Pietists within decades of their inception.
In Sweden, Pietist conventicles – the concept brought from Germany – existed as early as 1689, rousing similar opposition. The Conventicle Act of 1726 forbade all conventicles conducted by laypeople, though private devotional meetings under the direction of the clergy were permitted. The law was not repealed until 1858 in Sweden and 1870 in Finland, which was part of Sweden until 1809. In Finland, the conventicle has remained the base activity, especially in the Finnish Awakening revivalist movement.
In Russia, conventicles were held with the Orthodox Church in a position of ecclesiastical supremacy recognized by the State. Measures of repression were from time to time directed by the Government against dissenting sects which have incurred its suspicion and hostility, such as the Old Believers, Stundists, and Doukhobors, who were denied the liberty of private meetings for worship. The spirit of tolerance seemed to be rapidly gaining ground for a while, and nonconformists of any kind, on giving satisfactory assurances to the police, were generally permitted the liberty of worship according to their accepted mode.
Japanese Christian pacifist Uchimura Kanzō founded the Non-church movement in 1901. By 1979 about 35,000 people belonged to the movement, which had spread from Japan to Taiwan and South Korea.
The growth of conventicles is closely related to Pietism and the Charismatic movement. In the American Lutheran Church, in particular, there has been considerable debate about conventicles stemming from 17th- and 18th-century Pietism. Thompson attributes today's Lutheran mission societies, ladies' societies, youth groups, Bible studies, group devotions beyond actual church services, Lutheran elementary schools and high schools, and charitable and fraternal organizations associated with the church as conventicles. According to C. F. W. Walther, the founder of the Lutheran Church–Missouri Synod, such movements had to be opposed or carefully monitored, a caution that remains in effect.
According to Neville, the tradition of conventicles in Celtic lands can be found in the outdoor worship quite common in American communities, such as the South, populated by Celtic ancestors. Neville describes it as a folk tradition and ritual. Among the forms, conventicles take are frontier revivals, family reunions, and cemetery services as well as the more recent house church movement.
Conventicles in other religions
According to Smith, the mosque is a conventicle rather than an ecclesiastical institution. The mosque is an initiative of the community rather than a body led by a priesthood. In particular the Jama'at Khana (or musallah) approximates the status of a conventicle. According to Kaufman, modern-day Jewish synagogues resemble churches, whereas smaller meeting places—the shul, hevre, anshe, or shtibl—can be described as conventicle settings.
Early Mormon meetings were sometimes referred to as conventicles.
^Wilfred C. Smith. On Understanding Islam: Selected Studies. Walter de Gruyter, 1981. ISBN3110825805. p. 241
^David Kaufman (1999). Shul with a Pool: The Synagogue-Center” in American Jewish History. Hanover, N.H.: University Press of New England. P. 168.
^John G. Whittier (1848), "A Mormon Conventicle." In The Latter-day Saints Millennial Star, Volume 10, Publisher P.P. Pratt, 1848. Original from UC Southern Regional Library Facility
Digitized Feb 7, 2014
A conventicle is defined as an assembly of more than five persons besides the family meeting for worship not according to the Church of England. ---The History of England in Chronological Form. F.T. Jones, 1887.
16 Car. II., cap. 4, "An Act to prevent and suppresse seditious Conventicles." It was enacted that anyone of the age of sixteen or upwards present at an unlawful assembly or conventicle was to incur fine or imprisonment. A conventicle was defined as an assembly of more than five persons besides the members of a family met together for holding worship not according to the rites of the Church of England. The act was amended 22 Car. II., cap. 1 (1670), and practically repealed by the Toleration Act of 1689, but the act 22 Car. II., cap. 1, was specially repealed 52 Geo. III., cap. 155, s. 1. ---Wheatley, 1893.
‘conventicle, n. < Latin . . . . 4. a. A religious meeting or assembly of a private, clandestine, or illegal kind; a meeting for the exercise of religion otherwise than as sanctioned by the law. . . a1684 J. Evelyn Diary anno 1656 (1955) III. 181 To Lond, to receive the B: Sacrament, & was the first time that ever the Church of England was reduced to a Chamber & Conventicle, so sharp was the Persecution.
. . b. spec. in Eng. Hist. A meeting of (Protestant) Nonconformists or Dissenters from the Church of England for religious worship, during the period when such meetings were prohibited by the law.
This specific application gradually became distinct after 1593, and may be said to have been recognized by the ‘Conventicle Act’ of 1664; for although the word there occurs in constant conjunction with assembly and meeting, and always with qualification, it was entitled ‘An Act to prevent and suppress seditious conventicles’, by which title it is cited in the Act of Toleration of 1689.
The application to Nonconformist worship after its legalization or ‘establishment’ in 1689, and esp. after the repeal of the Conventicle Act in 1812, comes, according to circumstances, from a historical survival of the idea of illegality or from a living idea of schism or heresy. . . 1663 S. Pepys Diary 27 May (1971) IV. 159 The first [bill]..is, he [sc. Roger Pepys] says, too devilish a severe act against conventicles. 1664 Act 16 Chas. II c. 4 (Conventicle Act) Any Assembly Conventicle or Meeting under colour or pretence of any Exercise of Religion in other manner than is allowed by the Liturgy or practise of the Church of England. 1664 S. Pepys Diary 7 Aug. (1971) V. 235 Came by several poor creatures, carried by by Constables for being at a conventicle.’
Chart showing the number of references in each month of the diary’s entries.