St. Thomas preached the gospel wherever he went, and founded churches, Including one at Palayur of this Archdiocese. The apostolate of St. Thomas was fruitful among the people of Kerala. According to Malabar tradition seven Christian communities were formed in seven different places before he was martyred in 72 A.D. His body was brought to Mylapore and buried in a holy Shrine.
From the 4th century, St. Thomas Christians established relationship with the Church of Persia. Bishops who came to India from that Church also had their residence at Kodungalloor while they held the title of the "Metropolitan and gate of all India". The 'Metropolitan' changed his residence from Kodungalloor to Angamaly in the 16th Century due to certain Political reasons. After the death of Mar Abraham in 1588 St. Thomas Christians were subjected to the Latin Bishops from 1699 and continued to be so until 1887.
H.H. Pope Leo XIII of illustrious memory by the epoch making bull "Quod Jam Pridem" of May 20, 1887, separated the Syrian Catholics of Malabar from the Jurisdictions of Padroado as well as the Vicariate of Verapoly and erected for them two Apostolic Vicariates Trichur and Kottayam. The Vicariate of Trichur had within its territory two of the most ancient Christian centres ie, Kodungalloor and Palayoor, the former being the cradle of Christianity in India. Msgr. Adolphus Edwin Medlycott was the first Vicar Apostolic of Trichur.
Nine Years later, by the Brief "Quae Rei Sacrae" of July 28, 1896 the same Holy Father reorganised the two Vicariates into three: Trichur, Ernakulam and Changansserry with Indians of the Syro Malabara rite as their Vicars Apostolics Mar John Menacherry became the first native Vicar Apostolic of Trichur.
The rapid and remarkable progress that the Syrian Catholics made under the Indian bishops in the course of a few years was highly appreciated by the Holy See and as a result, on December 21, 1923 by the Bull "Romani Pontifices" of Pope Pius XI, the Syro - Malabar Hierarchy was established, raising Ernakulam to the status of a Metropolitan See and having Trichur, Changanaserry and Kottayam as its suffragan Eparchies. Mar Francis Vazhappilly was the First Bishop of Trichur.
In 1887 the Vicariate of Trichur was bounded on the East by the Western Ghats, on the south by the Periyar (Aluva), on the West by the Arabian Sea and on the North by the river Bharatha. When by "Quae Rei Sacrae" of July 28, 1896 the three Syrian Vicariates were erected, the river Chalakudy was made southern boundary of Trichur Vicariate.
Pope Pius XII, by the Bull "Saepe Fidelis" of April 29, 1955 extended the jurisdiction of the bishop of Trichur to the whole of the Diocese of Coimbatore. On June 20, 1974, Pope Paul VI by the Bull "Apostolico Requirente" divided the Eparchy of Trichur erecting the Eparchy of Palakkad which consisted of the civil districts of Palakkad and Coimbatore. Again on June 22, 1978, Pope Paul VI separated the entire taluk of Kodungalloor, greater part of Mukundapuram and small portions of Paravoor and Aluva to the north of the river Chalakudy from the Eparchy of Trichur and erected the Eparchy of Irinjalakuda by the Apostolic Bull "Trichuriensis Eparchiae".
Pope John Paul II inaugurated the centenary celebrations of the Eparchy of Trichur on February 7, 1986 at St. Thomas Nagar Trichur, in the presence of about one million people. On May 20, 1987 the centenary day was celebrated. The concluding functions on 9th August 1987 were presided over by Simon Cardinal Lourdusamy the then Prefect of the Congregation for the Eastern Churches. The eparchial renewal project - Evangelisation Decade - was inaugurated on December 30, 1990.
The long awaited expectation of the faithful of Trichur Eparchy was fulfilled on May 18, 1995 when the Pope John Paul II raised Eparchy to the Metropolitan status, Eparchies of Irinjalakuda and Palakkad as its suffragans and Bishop Mar Joseph Kundukulam as its first Metropolitan Archbishop.
The present Archieparchy of Trichur has the following boundaries - North: Bharathapuzha in Malappuram district and Trichur - Palakkad district boundary; West: Arabian Sea; East: Trichur district boundary; South: Southern boundary of Valapad village and Karuvannur, Karanchira, Kurumali, Mupliyam river