Thomas the Apostle , also known as Didymus ("twin"), was one of the Twelve Apostles of Jesus according to the New Testament. Thomas is commonly known as "Doubting Thomas" because he initially doubted the resurrection of Jesus Christ when he was told of it (as is related in the Gospel of John alone); he later confessed his faith ("My Lord and my God") on seeing the wounds left over from the crucifixion.

According to traditional accounts of the Saint Thomas Christians of modern-day Kerala in India, Thomas travelled outside the Roman Empire to preach the Gospel, travelling as far as the Tamilakam which is in South India, and reached Muziris of Tamilakam (modern-day North Paravur and Kodungalloor in Kerala State, India) in AD 52. In 1258, some of the relics were brought to Ortona, in Abruzzo, Italy, where they have been held in the Church of Saint Thomas the Apostle. He is regarded as the patron saint of India among its Christian adherents, and the Feast of Saint Thomas on July 3 is celebrated as Indian Christians' Day. The name Thomas remains quite popular among the Saint Thomas Christians of the Indian subcontinent.

3 July Latin Catholic Church, Liberal Catholic Church, Anglican Communion, Malankara Orthodox, Malankara Mar Thoma Syrian Church, Syro Malabar Church, Syro-Malankara Catholic Church, Believers Eastern Church, Syriac Catholic
21 December – Anglican Communion, Hispanic church, Traditional Catholics
26 Pashons and Sunday after Easter Thomas Sunday Coptic Christianity
6 October and Sunday after Easter Thomas Sunday – Eastern Orthodox