St. Thomas Christians (called Nasrani--Nazarenes--in Hindu) are groups of Christians in India whose roots go to the Malabar coast (now Kerala) in the first century CE and the seven churches that are believed to have been established by St. Thomas.
There he began preaching the Gospel. His teachings were accepted not only by those who chose to become Christians but also by those who chose to remain Hindus. The teachings eventually got integrated into the beliefs and traditions of the local communities, into their family history, into their songs and dances. St. Thomas established seven Christian communities or churches in Kerala. They are in Cranganore, Paravur(Kottakavu), Palayoor, Kokkamangalam, Malayattoor, Niranam, Chayal (Nilackal) and Kollam (Quilon).
In the 4th century CE, a settlement of Jewish Christians was founded in Kottayam by Thomas Kynai at the behest of the Catholicos of the East. They became known as the Knanaya. Currently, there are no independent Knanaya religious organizations but look either to the Pope of Rome or the Syrian Orthodox Church for ecclesiastic legitimacy. Nevertheless, this community has rigidly maintained its distinct ethnic identity to the present day, vehemently prohibiting intermarriage with non-Knanaya even within their own religious jurisdiction while freely permitting Knanaya marriage between Orthodox and Roman Catholic churches.
St. Thomas Christians were considered high caste, along the Hindu tradition, with special privileges granted by the kings. The archdeacon was the head of the Church, and Palliyogams (Parish Councils) were in charge of temporal affairs. They had a liturgy-centered life with days of fasting and abstinence. Their devotion to the St. Thomas Cross was absolute. Their churches were modelled after Hindu temples.
In short, the St. Thomas Christians of Kerala had blended well the ecclesiastical world of the East Syrian Church with the socio-cultural environment of their homeland. Thus, the East Syrian Church was Hindu in culture, Christian in religion and Syro-Oriental in worship. While the last trait may vary greatly in the present day among different heirs to the Nasrani tradition, their cultural identity as Hindu has not been lost.
According to Joas de Castro, the Portuguese Viceroy in Goa in 1548, the sword of the Portuguese was wielded "mainly against the centuries-old Christians of Kerala". This was because only in Kerala did the laity stand steadfast against Western colonization, and maybe the Portuguese, who were under the Roman Church, considered everything outside Roman as heretic.
The move against the Syrian Church was followed by Western Church establishing a European diocese in Goa in 1534. In 1557, Pope Paul IV declared Goa an archdiocese with its supremacy extending from the Cape of Good Hope at the southern tip of Africa to China, and all Christians, including the East Syrian Church, brought under its jurisdiction. The East Syrian Archdiocese of Angamali then became a dependent of Goa.
This Europeanization process led to divisions in the Church, as there was considerable resistance against Western domination. The Christian communities then split into many groups - East Syrian Roman Catholics, West Syrian Roman Catholics, Syrian Orthodox, Jacobite Syrian Orthodox, Mar Thoma (those who accepted the Anglican Church and other Protestant bodies), Church of the East (those who accepted the Nestorian Patriarch), and the Latin-Rite Roman Catholic Church.