The Mar Thomas Syrian Christian Community of Kerala was an autonomous Church in the unity of the Catholic Church, ever since its foundation in 52 A.D. by St. Thomas, the Apostle. They remained one body and flourished in the course of years, But in the seventeenth century, due to the intervention of the Portuguese fathers, a schism occurred. A good section of the Syrians eventually became Jacobites under the Patriarch of Antioch.
Ever since the schism, attempts at reunion were made. On September 20, 1930, the Reunion Movement took shape with the reunion of two eminent Jacobite Prelates the late Archbishop Mar Ivanios and late Bishop Mar Theophilos. It was a triumph of ecumenism, as their reunion took place as a Church, accepted with its liturgy, customs and jurisdiction. Thus it prepared a way for a complete reunion of the Jacobite-Orthodox Church of Kerala.
As the Reunion Movement developed speedily, Pope Pius XI established the Malankara Hierarchy for reuniting the people and created the Archdiocese of Trivandrum and the Diocese of Tiruvalla, by the Apostolic Constitution, "Christo Pastorum Principi", dated June 11, 1932.
Accordingly the Diocese of Tiruvalla was erected on November 6, 1933 and Bp Jacob Mar Theophilos was enthroned as its First Bishop. The diocese extended from the River Pampa in the south to the River Ponnani in the north and from the Arabian Sea in the west to the Western Ghats in the east. The Diocese was further extended to the north from the River Ponnani by the Decree of the Sacred Congregation for Oriental Churches by Prot. N. 534/43. dated February 14, 1958 and brought under it the whole Malabar region of Kerala the civil districts of Coimbatore and Nilgiris, and Karoor taluk in Tiruchirappally district of Tamil Nadu and the districts of Mysore, Mandya, Coorg, Hassan, Chickmangalur, Shimoga and South Kanara of Karnataka.
Pope John Paul II, by the Apostolic Constitution of October 28, 1978, separating the civil districts of Malappuram, Kozhikode and Canannore of Kerala ,Niligiris of Tamil Nadu and Coorg, Mysore, Mandya, Hassan, South Kanara, Chickmanglur and Shimoga of Karnataka from the Diocese of Tiruvalla, created the new Diocese of Bathery which was inaugurated on February 2, 1979.
Pope John Paul II sent His Eminence Cardinal Wladyslaw Rubin, Prefect of the Sacred Congregation for the Oriental Churches, as his delegate to the Golden Jubilee Celebration of the Reunion Movement, which was held in December 1980 at Kottayam, in the Diocese of Tiruvalla. The Golden Jubilee of the Establishment of the Diocese was celebrated in January 1984 at St. Johns Cathedral Tiruvalla. His Eminence Achille Cardinal Sylvestrini, the Prefect of the Oriental Congregation visited the Diocese in February 1996.