Gregory the Great
Catena Aurea · Johannesevangelium 20
And after eight days again his disciples were within, and Thomas with them: then came Jesus, the doors being shut, and stood in the midst, and said, Peace be unto you, Then saith he to Thomas, Reach hither thy finger, and behold my hands; and reach hither thy hand, and thrust it into my side: and be not faithless, but believing. And Thomas answered and said unto him, My Lord and my God. Jesus saith unto him, Thomas, because thou hast seen me, thou hast believed: blessed are they that have not seen, and yet have believed. And many other signs truly did Jesus in the presence of his disciples, which are not written in this book: But these are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name. Jesus saith unto him, Because thou hast seen Me, thou hast believed.
(Hom. xxvi.) But when the Apostle says, Faith is the substance of things hoped for, the evidence of things not seen, (Heb. 11:1) it is plain that things which are seen, are objects not of faith, but of knowledge. Why then is it said to Thomas who saw and touched, Because thou hast seen Me, thou hast believed? Because he saw one thing, believed another; saw the man, confessed the God. But what follows is very gladdening; Blessed are they that have not seen, and yet have believed. In which sentence we are specially included, who have not seen Him with the eye, but retain Him in the mind, provided we only develope our faith in good works. For he only really believes, who practises what he believes.