John Calvin on John 20:24

24. But Thomas, one of the twelve. The unbelief of Thomas is here related, that by means of it the faith of the godly may be more fully confirmed. He was not only slow and reluctant to believe, but even obstinate. His dulness of apprehension was the reason why Christ again permitted them both to see and to feel him, in the same manner as before. In this manner, a new addition to the proof of Christ’s resurrection was given, not only to Thomas, but, also to us. Besides, the obstinacy of Thomas is an example to show, that this wickedness is almost natural to all men, to retard themselves of their own accord, when the entrance to faith is opened to them.