The Synod of Diamper, held at Udayamperoor/Diamper, (Kerala, India) is a diocesan synod by which Latin usages were formally adopted by the Christians of Saint Thomas. It was convened on June 20, 1599 under the leadership of Aleixo de Menezes, Archbishop of Goa. Archdeacon George (of the Cross) was forced to comply with the wishes of Archbishop of Goa. This separated the Thomas Christians from the Chaldean Patriarch and subjected them directly to the Latin Archbishopric of Goa. The Archbishopric of Angamale was downgraded to a Bishopric under Goa in 1600 AD. Portuguese Padroado rule was thus imposed and the Bishops for Saint Thomas Christians were appointed by Portuguese Padroado.
In 1597, Mar Abraham, the last metropolitan archbishop appointed by the Chaldean Patriarch, died. His Archdeacon, George (of the Cross) according to the custom and by virtue of appointment of Mar Abraham, took up the administration of the Archdiocese of Angamale. Menezes hastened to Nominate Fr. Francis Ros SJ as Administrator. In the end since the Archdeacon George (of the Cross) was well accepted by the people, Menezes had unwillingly reverse his decesion and confirm the Archdeacon as Administrator. Archdeacon called together an assembly of the Saint Thomas Christians at Angamali, in which a solemn oath was taken that they would act only according to the wishes of Archdeacon, and that if the Pope sent them as their Bishop, not their Archdeacon, but a Latin, that they would plead their case with Rome.
Menezes undertook a visit to all the Churches of Saint Thomas Christians in February 1599, which lasted for few months and slowly earned the good will of people. After having won over a considerable number of people and priests, Menezes threatened to depose Archdeacon, George (of the Cross) and appoint in his place Thomas Kurian, another nephew of former Archdeacon whose claims had been ignored in 1593. In order to prevent a division, Archdeacon, George (of the Cross) yielded to the demands of Menezes.
This void in leadership in India cleared the way for the Portuguese to impose their own customs, hierarchy, law, liturgy and rites among the Saint Thomas Christians. Many of the local customs were officially anathematised as heretical and their manuscripts were condemned to be either corrected or burnt.
Dom Alexis Menezes, Archbishop of Goa, summoned all the priests, other clerics and four lay men elected from each church, even from the churches he had not visited under the pain of excommunication. About 130 ecclesiastics and 660 laymen ( elected and specially invited) met at Diamper in the territory of Kingdom of Cochin. 
The Syond solemnly began on the third Sunday after Pentecost, 20th June, 1599. It was held in the church of Diamper ( Udayamperoor) from June 20 to 26, 1599. Archbishop Menezes presided the Syond. The Chaldean Patriarch who was in communion with Rome was condemned as a heretic and schismatic, and they were made to swear that they would not accept any bishop except the one immediately nominated by Rome. The controversial Syond of Diamper canonized Romanization of the Church of Saint Thomas Christians. Aleixo de Menezes, laboring under the shadow of the Protestant Reformation and the Council of Trent, was unwilling to give an inch to the customs of the Saint Thomas Christians.
Decrees of the Synod
The Synod issued 200 decrees distributed in nine actions (sessions). There are some differences exist among the decrees of the Synod which are available today. It has been suggested that these degrees were first formulated in Portuguese language by Dom Menezes and then translated to Malayalam. It has been suggested that the participants were made to sign the Malayalam document which lack 35 Cannons given in the Portuguese text. 
Changes in Liturgy
The text on which the Synod worked was a composite East Syriac text of Anaphora of Addai and Mari.  The Synod declared certain passages of the Holy Qurbana of Addai and Mari as impious, sacrilegious and outcome of Nestorian heresy. The changes made by the Synod are six in litanies, seven in hymns or anthems, four in formulae pertaining to the deacon, one in response of the people, one in the text of the gospel lesson, one in affecting the whole creed. In the paryer of the priest there are five changes in pre- anaforaml part of the Qurbana of Addai and Mari. There are four changes with in the anaphora and eleven in the four variables hutame (Sealing prayers). 
Changes in Administration
Under Portuguese Padraodo, Latin Bishops were appointed to govern the Saint Thomas Christians. Fr. Franics Roz SJ was nominated as successor to Mar Abraham on Nov 5, 1599. Bishop Roz SJ, centralized in himself all the authority reducing almost to nothing the powers of Archdeacon. Bishop Franics Roz SJ died on Feb 16, 1624 and was succeeded by Bishop Britto Stephen SJ. Archdeacon George (of the Cross) died c 1634 and was succeeded by Archdeacon Thomas. Bishop Britto SJ died in 1641 and Bishop Garcia Franics SJ succeeded him. A regular fight ensued between the new Archbishop Garcia Franics SJ and the new Archdeacon Thomas. 
Destruction of Syriac Books.
The decree XVI ordered that all the Syriac MSS should be handed over to the Archibishop or his deputy on visit to the Churches. Due to the lack of printed books, the Qurbana MSS were excluded from this.
Some of the other books which are said to be burnt at the synod of Diamper are, 1.The book of the infancy of the savior (history of our Lord) 2. Book of John Braldon 3. The Pearl of Faith 4. The Book of the Fathers 5. The Life of the Abbot Isaias 6. The Book of Sunday 7. Maclamatas 8. Uguarda or the Rose 9. Comiz 10. The Epistle of Mernaceal 11. Menra 12. Of orders 13. Homilies (in which the Eucharist is said to be the image of Christ) 14. Exposition of Gospels. 15. The Book of Rubban Hormisda 16. The Flowers of the Saints 17. The Book of Lots 18. The Parsimon or Persian Medicines 
There are only very few Syriac MSS which withstood the destruction.
Invalidity of the Synod
A number of studies has questioned the invalidity of the Synod of Diamper starting with the studies of Bishop Jonas Thaliath. Thaliath in his thesis at the Pontifical Gregorian University in 1952 proved the synod as invalid on the ground that it was convoked without authority, conducted not according to the Canon of the Church and was never properly approved by Rome. 
In 1975, a dissertation on the same subject with the title "The disciplinary Legislation of Synod of Diamper" was submitted to the faculty of Canon Law of Oriental Institute, by Joseph Kuzhinjalil, treating the invalidity of the Synod of Diamper as an established fact. He further demonstrated that most of the disciplinary norms of the synod were fruit of ignorance concerning the Eastern Liturgico - sacramental discipline and the indigenous customs and traditions of St. Thomas Christians. 
The Synod of Diamper was a turning point in the history of Church of Malabar. It gave a definite form and set up to the tendency of Latinization that slowly prevailed in the Church since the arrival of Portuguese missionaries. It removed some of the abuses arising out of the ignorance of the people but at the same time severed the age long relation this Church had with that of Babylon.
The result of the synod, however, was unfortunate. As the Catholic Encyclopedia (1913) says:
The only case in which an ancient Eastern rite has been wilfully 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.
Some time after the Synod of Diamper, on 25 November 1599, a letter was send to Pope by Archdeacon, giving information about the Synod and its work. The letter praises the work of Menezes and request the appointment of Menezes or Fr. Francis Roz as their Bishop . The letter hardly represent the genuie sentiments of Archdeacon as by that time he was completely at the mercy of the will of Portuguese and the only thing left for him was to follow their directives.
In this way the Synod of Dimaper achieved one of the aims of the Portuguese policy in Kerala, to separate the Syrian Christians of Malabar from the Chaldean Patriarch and to extend the influence of Portuguese Padrado in India. As a result, the King of Portugal, got the right of nomination to the ancient See of Saint Thomas in Malabar. The Archbishopirc of Angamale was degraded to a Portuguese Padraodo diocese under Goa in August,4th 1600 AD.
The oppressive rule of the Portuguese padroado provoked a violent reaction on the part of the indigenous Christian community. The first solemn protest took place in 1653, known as the Koonan Kurishu Satyam (Koonan Cross Oath). Under the leadership of Archdeacon Thomas, a part of the Thomas Christians publicly took an oath in Matancherry, Cochin, that they would not obey the Portuguese bishops and the Jesuit missionaries. In the same year, in Alangad, Archdeacon Thomas was ordained, by the laying on of hands of twelve priests, as the first known indigenous Metropolitan of Kerala, under the name Mar Thoma I. Those who took, or supported, the Oath became the Puthenkoottukar or New Party, while those who remained faithful to the Catholic Church became the Pazhayakoottukar or Old Party.
After the Coonan Cross Oath, between 1661 and 1662, out of the 116 churches, the Old Party reclaimed eighty-four churches, leaving Archdeacon Mar Thoma I only thirty-two churches. The eighty-four churches and their congregations were the body from which the Syro Malabar Catholic Church have descended. The other thirty-two churches and their congregations were the body from which the Syriac Orthodox (Jacobites & Orthodox), Thozhiyur (1772), Mar Thoma (Reformed Syrians) (1874), Syro Malankra Catholic Church have originated.  In 1665, Mar Gregorios Abdul Jaleel, a Bishop send by the Syriac Orthodox Patriarch of Antioch arrived in India and the dissident group under the leadership of the Archdeacon welcomed him.  This visit resulted in the Mar Thoma party claiming spiritual authority of the Antiochean Patriarchate and gradually introduced the West Syrian liturgy, customs and script to the Malabar Coast.
Though most of the Thomas Christians gradually relented in their strong opposition to the Western control, the arrival of Mar Gregorios in 1665 marked the beginning of a formal schism among the Thomas Christians. Those who accepted the West Syrian theological and liturgical tradition of Mar Gregorios became known as Jacobites. Those who continued with East Syrian theological and liturgical tradition and stayed faithful to the Synod of Diamper are known as the Syro-Malabar Catholic Church in communion with the Catholic Church. They got their own Syro-Malabar Hierarchy on 21 December 1923 with the Metropolitan Mar Augustine Kandathil as the Head of their Church.
St. Thomas Christians by this process got divided in to East Syrians and West Syrians.
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 CHAI, Volume II, History of Christianity in India p-70-75
- ↑ Syond of Diamper, NSC Network.
- ↑ Syond of Diamper Church, Garvasis and Protasis church and All Saints church.
- ↑ CHAI, Volume II, History of Christianity in India p-70-75
- ↑ J Thaliath, The Syond of Diamper
- ↑ J Thaliath, The Synod of Diamper
- ↑ D. Webb, “ Versions of the Malabar Liturgy”
- ↑ Connolly, “ Work of Menezes”, Codrington, “ The Malabar Liturgy and the Synod of Diamper”
- ↑ Synod of Diamper, NSC Network.
- ↑ Ferroli, "Jesuits in Malabar" Vol.1
- ↑ Jonas Thaliatth, 'The synod of Diamper (1958)
- ↑ Paul Pallath, The Synod of Diamper : valid or invalid?, The Synod of Diamper Revisited, Edited by George Nedungatt S.J., Pontifico Instituto Orientale, Rome, 2001, p. 199
- ↑ Jonas Thaliath, The Synod of Diamper, P-31
- ↑ Giuseppe Beltrami, La Chiesa Caldea pp 253-6, Full text reproduced
- ↑ Catholic Encyclopedia- “St. Thomas Christians” The Carmelite Period,Dr. Thekkedath, History of Christianity in India”
- ↑ Claudius Buchanan 1811 ., Menachery G; 1973, 1982, 1998; Podipara, Placid J. 1970; Leslie Brown, 1956; Tisserant, E. 1957; Michael Geddes, 1694;
Books (Studies on Synod of Diamper)
- Paul Pallath, "The Synod of Diamper : valid or invalid?"
- George Nedungatt S.J.,"The Synod of Diamper Revisited", Pontifico Instituto Orientale, Rome, 2001.
- Joseph Kuzhinjalil, "The disciplinary Legislation of Synod of Diamper" (1975)
- Jonas Thaliath, " The Synod of Diamper" (1958)
- Connolly, " The Work of Meneses"
- Codrington, " The Chaldean Liturgy"
- Codrington," The Malabar Liturgy and Synod of Diamper"
- Udayamperoor Synod Church
- Pontifical Council For Promoting Christian Unity
- History of Udayamperoor Synodal Church in Malayalam
- History of the Syro-Malabar Church
- Catholic Encyclopedia: St. Thomas Christians (Archbishop Menezes and the Synod of Diamper)
- Catholic Encyclopedia: Rites