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The Coonan Cross Oath was taken in 1653, by a part of Saint Thomas Christians, reacting to the persecution of their Church by the Portuguese colonials who sought to bring it under Portuguese Padroado or Propaganda Fide. Those who swore the oath vowed that neither they or their descendants to come would have anything to do with the Portuguese Padroado and that they would never bow down before them.
The Saint Thomas Christians remained in communion with the Assyrian Church of the East until their encounter with the Portuguese in 1498. With the establishment of Portuguese power in parts of India, clergy of that nationality, in particular certain members of the Society of Jesus (Jesuits), attempted to Latinize the Indian Christians.
The Portuguese started a Latin diocese in Goa (1534) and another at Cochin (1558) in the hope of bringing the Thomas Christians under their jurisdiction. In a Goan Synod held in 1585 it was decided to introduce the Latin liturgy and practices among the Thomas Christians. In the Synod of Diamper of 1599 the Portuguese Archbishop, Don Alexio de Menezes, succeeded in appointing a Latin bishop to govern the Thomas Christians. The Portuguese Padroado was extended over them.
The Portuguese refused to accept the legitimate authority of the Indian hierarchy and its relation with the Assyrian East Syrians. At the synod held in Diamper in 1599, the Portuguese Archbishop of Goa imposed a large number of Latinizations. The Church of Malabar came under a forced communion with Rome. From 1599 up to 1896 these Christians were under the Latin Bishops who were appointed either by the Portuguese Padroado or by the Roman Congregation for the Propagation of the Faith (Propaganda Fide). Every attempt to resist the Latinization process was branded by them heretical. Under their indigenous archdeacon, the Thomas Christians resisted, but the result was disastrous.
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.
- ↑ "Koonan Oath 00001". http://126.96.36.199/search?q=cache:grrXjMjR0a8J:www.education.kerala.gov.in/englishmedium/historyeng/chapter8.pdf+coonan+cross+oath&hl=en&ct=clnk&cd=5&gl=us. Retrieved 2007-05-20.
- ↑ 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;
- ↑ Dr. Thekkedath, History of Christianity in India”