Thomas is the apostle associated with the label “Doubting Thomas” because of his reaction to Jesus’s resurrection in the Gospel of John. Thomas says he will not believe that Jesus is alive unless he sees the wounds and touches them.
When Jesus later appears to the disciples, Thomas is present. Jesus invites him to examine the wounds, and Thomas responds with the confession, “My Lord and my God!” The passage presents Thomas’s doubt alongside a movement toward belief rather than simply as a permanent character flaw.
Thomas is also called Didymus in John; both names mean “twin” in different languages. Later Christian traditions connect him with missionary activity far beyond the New Testament, especially in India, but those traditions are separate from the specific resurrection scene in John 20. The nickname itself comes from this famous episode.