“The other disciples therefore said unto him, We have seen the Lord. But he said unto them, Except I shall see in his hands the print of the nails, and put my finger into the print of the nails, and thrust my hand into his side, I will not believe.”
So the other disciples told him, 'We have seen the Lord.' But he said to them, 'Unless I see the mark of the nails in his hands and put my finger in the mark and put my hand in his side, I will not believe' — Thomas demands empirical, tactile proof; his list of verification (seeing nails, touching wounds, placing hand in side) mirrors but exceeds the seeing of others. His condition is not faithlessness but the demand that resurrection be bodily, not phantom. His refusal to accept the disciples' testimony places him at the Gospel's crisis point: faith through witness versus faith through direct encounter.
Church Fathers on John 20:25
The disciples, when they heard what Mary told them, were obliged either to disbelieve, or, if they believed, to grieve that He did not count them worthy to have the sight of Him. He did not let them however pass a whole day in such reflections, but in the midst of their longing trembling desires to see Him, presented Himself to them: Then the same day at evening, being the first day of the week, when the doors were shut where the disciples were assembled for fear of the Jews.
Wherein is strewn the infirmity of the Apostles. They assembled with doors shut, through that same fear of the Jews, which had before scattered them: Came Jesus, and stood in the midst. He came in the evening, because they would be the most afraid at that time.
and with shut doors, that he might show how that in the very same way he had risen again, i.e. with the stone lying on the sepulcher.