'My Lord and my God'
Whether Thomas addresses Jesus as God or exclaims in surprise.
John 20:24-29
24But Thomas, one of the twelve, called Didymus, wasn’t with them when Jesus came.
25The other disciples therefore said to him, “We have seen the Lord!” But he said to them, “Unless I see in his hands the print of the nails, put my finger into the print of the nails, and put my hand into his side, I will not believe.”
26After eight days again his disciples were inside and Thomas was with them. Jesus came, the doors being locked, and stood in the middle, and said, “Peace be to you.”
27Then he said to Thomas, “Reach here your finger, and see my hands. Reach here your hand, and put it into my side. Don’t be unbelieving, but believing.”
28Thomas answered him, “My Lord and my God!”
29Jesus said to him, “Because you have seen me, you have believed. Blessed are those who have not seen, and have believed.”
What each tradition says
Rabbinic / Midrashic
Kohler says the disciples and the women who had been Jesus’ constant companions while he was alive beheld him in their entranced state as partaking of their meals. They heard him address instruction and argumentation to them.
Kaufmann Kohler, The Jewish Encyclopedia (1905), art. "Jesus of Nazareth" — In Theology (the resurrection narratives)
Patristic
Augustine says Thomas saw and touched the man and acknowledged the God he neither saw nor touched. By means of what he saw and touched, Thomas put away every doubt and believed the other.
Augustine of Hippo (354-430), Tractates on the Gospel of John (trans. John Gibb), in Nicene and Post-Nicene Fathers, ser. 1, vol. 7 — Tractate 121, §5, on John 20:28
Catholic
Cornelius a Lapide says Thomas's words show that Jesus was very and true God by nature. Thomas humbly and penitently confesses and condemns his former incredulity, with a great profession of faith, hope, penitence, and love.
Cornelius a Lapide (1567-1637), The Great Commentary of Cornelius à Lapide — On John 20:28
Orthodox
Cyril of Alexandria says that He who is Lord by nature and Ruler over all is also God. He states that this follows with certainty and that all must confess it.
Cyril of Alexandria (c. 376-444), Commentary on the Gospel according to S. John, Vol. II (Library of Fathers; trans. T. Randell, 1885) — On John 20:28
Reformed / Protestant
Calvin writes that he plainly ratifies what Thomas said. Therefore, Calvin says, this passage is abundantly sufficient for refuting the madness of Arius, since it is not lawful to imagine two Gods.
John Calvin (1509-1564), Commentary on John, Volume 2 (CCEL text of the Calvin Translation Society edition) — On John 20:28
Evangelical
John Wesley reads Thomas’s words as confessing Jesus’s Godhead, not merely acknowledging him as Lord or risen. Thomas, Wesley says, confesses Jesus as God more explicitly than any other had yet, and does so without putting his hand upon his side.
John Wesley (1703-1791), John Wesley's Notes on the Bible — Notes on John 20:28