Coonan Cross Oath in the context of "Malankara Church"

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 Synod of Diamper (Udayamperoor Synod) (Malayalam: ഉദയംപേരൂർ സൂനഹദോസ്, romanized: Udayampērūṟ Sūnahadōs), held at Udayamperoor (known as Diamper in non-vernacular sources) in June 1599, was a diocesan synod, or council, that created rules and regulations for the ancient Saint Thomas Christians (also known as Mar Thoma Nasranis) of the Malabar Coast, a part of modern-day Kerala state, India, formally subjugating them and downgrading their whole Metropolitanate of India as the Diocese of Angamale, a suffragan see to the Archdiocese of Goa administered by Latin Church Padroado missionaries. This synod also introduced forced Liturgical Latinisation and the eschewal of local practices and beliefs, leading to a significant ecclesial protest by Saint Thomas Christians known as Coonan Cross Oath and a subsequent schism in the mid-17th century.

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.