On 20 June 1599, the Synod of Diamper began in Udayamperoor/Diamper in Kerala, India] under the leadership of Archbishop Aleixi de Menezes.*
When the Portuguese encountered the Thomas Christians after the arrival of explorer Vasco de Gama in 1498, the Thomas Christians were part of the Assyrian Church of the East or Chaldean Church.* As a result of Portuguese missionary activity many Thomas Christians were influenced by the rituals and practices of the Roman Catholic Church and in 1552 a group of Thomas Christians entered into communion with the Pope.
However, the Portuguese hierarchy in India wanted to bring the Thomas Christians into closer jurisdiction of the Latin hierarchy and replace the Assyrian/Chaldean liturgy with that of the Roman Catholic Church. Many of the local customs were condemned as heretical and many of the liturgical books of the Thomas Christians were ordered corrected or were burnt.
The Archbishop summoned all priests to the Synod of Diamper under pain of excommunication. 130 priests and 660 laymen met at the Synod which lasted until 26 June 1599. The Synod, presided by the Archbishop of Goa, condemned the Chaldean Patriarch who was in communion with Rome to be a heretic and a schismatic, Thomas Christians were not to accept any bishop except one immediately chosen by Rome, and the Latinization/adoption of Roman Catholic traditions and practices was confirmed.
More recently, the studies of Bishop Jonas Thaliath has demonstrated that the Synod of Diamper was invalid on the grounds that the Synod was convoked without the proper authority from Rome and did not follow Canon Law.
The effect of the Synod was to provide a greater push toward the further Latinization of the Thomas Christians and to separate them from their historic ties to the Chaldean Church. As the Catholic Encyclopedia notes, “The only case in which an ancient Eastern rite has been willfully romanized is that of the Uniat Malabar Christians, where it was not Roman authority but the misguided zeal of Alexius de Menezes, Archbishop of Goa, and his Portuguese advisers at the Synod of Diamper (1599) which spoiled the old Malabar Rite.”
Moreover, hostility from the Thomas Christians to the Portuguese treatment led to the Koonan Kurishu Satyam (Koonan Cross Oath) in 1653. At which some of the Thomas Christians swore that they would not obey the Portuguese bishops or the Jesuit missionaries. This will lead to a split among the Thomas Christians between the Syro Malabar Catholic Church which followed the Synod of Diamper and the Syriac Orthodox which did not.
The history of the Syro-Malabar Church
*The native Christians of India called themselves Thomas Christians because there tradition states that they were evangelized by Thomas the Apostle.