Synod of Diamper in the context of "Malankara Metropolitan"

The Saint Thomas Christians, also called Syrian Christians of India, Marthoma Suriyani Nasrani, Malankara Nasrani, or Nasrani Mappila, are an ethno-religious community of Indian Christians in the state of Kerala (Malabar region), who, for the most part, employ the Eastern and Western liturgical rites of Syriac Christianity. They trace their origins to the evangelistic activity of Thomas the Apostle in the 1st century. The Saint Thomas Christians had been historically a part of the hierarchy of the Church of the East but are now divided into several different Eastern Catholic, Oriental Orthodox, Protestant, and independent bodies, each with their own liturgies and traditions. They are based in Kerala and they speak Malayalam. Nasrani or Nazarene is a Syriac term for Christians, who were among the first converts to Christianity in the Near East.

Historically, this community was organised as the Province of India of the Church of the East, by Patriarch Timothy I (780–823 AD) in the eighth century, it was served by bishops and a local dynastic archdeacon. In the 14th century, the Church of the East declined in the Near East, due to persecution from Tamerlane. Portuguese colonial overtures to bring St Thomas Christians into the Latin Church of the Catholic Church, administered by their Padroado system in the 16th century, led to the first of several rifts (schisms) in the community. The attempts of the Portuguese culminated in the Synod of Diamper, formally subjugating them to the Portuguese Padroado and imposing upon them the Roman Rite of worship. The Portuguese oppression provoked a violent resistance among the Thomasine Christians, that took expression in the Coonan Cross Oath protest in 1653. This led to the permanent schism among the Thomas' Christians of India, leading to the formation of Puthankoor or Puthankūttukār ("New allegiance" ) and Paḻayakūṟ or Pazhayakūr ("Old allegiance") factions. The Paḻayakūṟ comprise the present day Syro-Malabar Church and Chaldean Syrian Church which continue to employ the original East Syriac Rite. The Puthankoottukar, who continued to resist the Catholic missionaries, organized themselves as the independent Malankara Church and entered into a new communion with the Syriac Orthodox Church of Antioch, inheriting from them the West Syriac Rite, replacing the old East Syriac Rite liturgy.

The Coonan Cross Oath (alternatively spelled Koonan Cross Oath), also known as the Great Oath of Bent Cross or Leaning Cross Oath, was taken on 3 January 1653, in Mattancherry, by a significant portion of the Saint Thomas Christian community in the Malabar region of India. This public declaration marked their refusal to submit to the authority of the Jesuits and the Latin Catholic hierarchy, as well as their rejection of Portuguese dominance in both ecclesiastical and secular matters.

The Portuguese had established political and religious control over parts of India, seeking to enforce Latin Catholic practices upon the local Christian community, which followed its own traditions. At the Synod of Diamper in 1599, led by Archbishop Aleixo de Menezes, the Western Church imposed several changes, including the use of Portuguese Archbishops, the Latin liturgy, Roman vestments, clerical celibacy, and the establishment of the Portuguese Inquisition in Goa, which also extended its jurisdiction to Cochin. The Saint Thomas Christians, whom the Portuguese referred to as "Nestorians," largely resisted these latinizations, as they were seen as an infringement on their longstanding religious customs.

The Paḻayakūṟ (Pazhayakoor; English: "Old Allegiance"), also known as Romo-Syrians or Syrian Catholics of Malabar, are the Saint Thomas Christians who use the East Syriac Rite and claim apostolic origin from the Indian mission of Thomas the Apostle in the 1st century AD.

The Saint Thomas Christians were originally in full communion with the Church of the East in Persia, from whom they inherited the East Syriac liturgical rite. Through the Schism of 1552, a faction of the Church of the East entered the Catholic Church. Following the 1599 Synod of Diamper, they were placed under the Latin Church's Padroado missionaries, who took over the jurisdiction of Angamaly. After the Coonan Cross Oath of 1653, which constituted a secession from the Padroado, the Paḻayakūṟ quickly returned to the Catholic Church as East Syriac Catholics under Archbishop Palliveettil Chandy.