Thomas has said he won't believe unless he can actually put his finger into the mark of the nails, thrust his hand into the place where the spear went into Jesus' side. And it would have been better if Thomas had believed without needing that, but Jesus meets him where he is. "OK, Thomas, here are my hands, here's my side, don't be faithless. But believe." And Thomas takes the flying leap of faith and doesn't just say, "OK, all right, I believe." He says what none of the others have said to this point, "My Lord and my God."
See what I mean? Thomas is called "doubting" Thomas—but he is the first one to make the connection. He got a bum rap in history; he should be celebrated as the first one who truly comprehended (after the resurrection) who Jesus was.