The ecclesiastical province came into existence on 31 December 1953 through the papal bullAd Christi Ecclesiam Regendam issued by Pope Pius XII. The boundaries of the new diocese were the same as those of the Latin Church Diocese of Calicut. Later, as Syro-Malabar Catholics migrated even to the states of Karnataka and Tamil Nadu, the boundaries were extended to the present Latin Diocese of Mangalore, Chickmangalur, Mysore, Shimoga, and Ootacamund by a decree of the Holy See on April 29, 1955.
Malabar Migration
The history of the Malabar Migration and that of the ecclesiastical province of Thalassery are closely connected. The Thalassery diocese took up the struggle of the settlers, who were mostly Syriac Rite Catholics not accustomed to Roman Rite ceremonies. Petitions were sent to the Holy See by the bishops of the settlers, who had migrated to the British Malabar since 1930, requesting for a diocese of their own. Cardinal Tisserant, the Prefect of the Oriental Congregation came to Malabar and personally saw the pitiable situation of the settlers. With the help of locals and priests, they started to make changes in the region.