The word “Catholicos” means “The General Head”. It can be considered as equivalent to “Universal Bishop”. There were only three ranks of priesthood in the early Church: Episcopos (Bishop), Priest, and Deacon. By the end of the 3rd century certain bishops of certain important cities in the Roman empire gained pre-eminence over other bishops and they came to be known as Metropolitans. The Ecumenical councils of the 4th century recognized the supreme authority of these Metropolitans. By the 5th century, the Bishops in major cities like Rome, Constantinople, Alexandria, Antioch etc. gained control over the churches in the surrounding cities. Gradually they became the heads of each independent regional church and were called Patriarch which means common father. The same rank in the Churches outside the Roman Empire was called Catholicos. There were three ancient Catholicates in the Church before the 5th century. They were the Catholicate of the East, the Catholicate of Armenia and the Catholicate of Georgia. None of these ranks and titles are the monopoly of any church. Any Apostolic and national church has the authority to declare and call its head, Catholicos, Pope, or Patriarch*.
St.Thomas established the church in India and is recognized as its first Head or Catholicos.
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