The Quinisext Council (often called the Council in Trullo, Trullan Council, or the Penthekte Synod) was a church council held in 692 at Constantinople under Justinian II. It is often known as the Council in Trullo, because like the Sixth Ecumenical Council it was held in a domed hall in the Imperial Palace (τρούλος [troulos] meaning a cup or dome). Both the Fifth and the Sixth Ecumenical Councils had omitted to draw up disciplinary canons, and as this council was intended to complete both in this respect, it took the name of Quinisext (Latin: Concilium Quinisextum, Koine Greek: Πενθέκτη Σύνοδος, Penthékti Sýnodos), i.e. the Fifth-Sixth Council. It was attended by 215 bishops, all from the Eastern Roman Empire. Basil of Gortyna in Crete, however, belonged to the Roman patriarchate and called himself papal legate, though no evidence is extant of his right to use that title.
Many of the Council's canons were reiterations. It endorsed not only the six ecumenical councils already held (canon 1), but also the Apostolic Canons, the Synod of Laodicea, the Third Synod of Carthage, and the 39th Festal Letter of Athanasius (canon 2).
The Council banned certain festivals and practices which were thought to have a pagan origin
. Therefore, the Council gives some insight to historians about pre-Christian religious practices.
Many of the council's canons were aimed at settling differences in ritual observance and clerical discipline in different parts of the Christian Church. Being held under Byzantine auspices, with an exclusively Eastern clergy, these overwhelmingly took the practice of the Church of Constantinople as orthodox. It explicitly condemned some customs of Armenian Christians – among them using wine unmixed with water for the Eucharist (canon 32), choosing children of clergy for appointment as clergy (canon 33), and eating eggs and cheese on Saturdays and Sundays of Lent (canon 56) – and decreed deposition for clergy and excommunication for laypeople who contravened the canons prohibiting these practices. Likewise, it reprobated, with similar penalties, the Roman custom of not allowing married individuals to be ordained to the diaconate or priesthood unless they vowed for perpetual continence (canon 13), and fasting on Saturdays of Lent (canon 55). Without contrasting with the practice of the Roman Church, it also prescribed that the celebration of the Eucharist in Lent should only happen in Saturdays, Sundays, and the feast of the Annunciation (canon 52).
While the Orthodox Church widely considers this council an addendum to the Fifth and Sixth Ecumenical Councils, adding its canons thereto , the Roman Catholic Church has never accepted the council as authoritative or in any sense ecumenical. In the West, Venerable Bede calls it (in De sexta mundi aetate) a "reprobate" synod, and Paul the Deacon an "erratic" one. For the attitude of the Roman bishops, in face of the various attempts to obtain their approval of these canons see Hefele. However, Pope Hadrian I did write favourably of the canons of this council.
The Pope of the time of the council, Sergius I, who was of Syrian origin, rejected it, preferring, he said, "to die rather than consent to erroneous novelties": though a loyal subject of the Empire, he would not be "its captive in matters of religion" and refused to sign the canons. Emperor Justinian II ordered his arrest and abduction to Constantinople by the notoriously violent protospatharios Zacharias. However, the militia of the exarchate of Ravenna frustrated the attempt. Zacharias nearly lost his life in his attempt to arrest Sergius I. Louis Duchesne suggests that it was in protest against the Council's banning of representations of Christ as a Lamb that Pope Sergius introduced the singing of the Agnus Dei at the breaking of the host at Mass.
In Visigothic Spain, the council was ratified by the Eighteenth Council of Toledo at the urging of the king, Wittiza (694 – probably 710), who was vilified by later chroniclers for his decision. Fruela I of Asturias (757–768) reversed the decision.