When the apostle Paul says that "faith is the ground of things to be hoped for, the proof of things that are not evident" (Hebrews 11:1), it is clear that faith is the proof of those things that cannot be made evident. Things that are evident no longer involve faith but recognition. Why then, when Thomas saw and when he touched, was it said to him, "Because you have seen me, you have believed"? Because he saw one thing, and he believed another; divinity could not be seen by a mortal person. He saw a human being, and he confessed him as God. . . . But we also rejoice at what follows, "Blessed are those who have not seen and have believed." Certainly this saying refers to us who keep in our minds one whom we do not see in his body. It refers to us, but only if we follow up our faith with our works. That person truly believes who expresses his faith in his works.From Gregory the Great, "Forty Gospel Homilies 26", as found in The Ancient Christian Commentary on Scripture, NT IVb: John 11-21 (p. 373).
Or, as this ACCS volume's editor, Joel C. Elowsky, subtitles the comments on John 20:29, "Seeing is not believing" or "Thomas sees one thing, believes another."
I just love reading the Fathers: "[H]e saw one thing, and he believed another; divinity could not be seen by a mortal person. He saw a human being, and he confessed him as God."