Thomas the Apostle (Imperial Aramaic: 𐡀𐡌𐡅𐡕𐡌, Biblical Hebrew: תוֹמא הקדוש or תוֹמָא שליחא (Toma HaKadosh "Thomas the Holy" or Toma Shlikha "Thomas the Messenger/Apostle" in Hebrew-Aramaic), Classical Syriac: ܬܐܘܡܐ, Tʾōmā, meaning "twin"; Koinē Greek: Θωμᾶς), also known as Didymus (Greek: Δίδυμος Didymos, meaning "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); 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, Saint 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.

Many churches in the Middle East and southern Asia, besides India, also mention Apostle Thomas in their historical traditions as being the first evangelist to establish those churches, the Assyrian Church of the East, the early church of Sri Lanka.