William Burkitt on John 20:29

Here we have Christ's reprehension of Thomas for not believing without such sensible evidence as he desired. He believed now that Christ was risen from the dead, but it was upon the testimony of his senses only. Therefore Christ tells him, that his faith would have been more excellent and more eminently rewardable, if he had believed without such demonstrative evidence: Faith is the evidence of things not seen. Therefore to give credit to a thing upon the evidence of a sense, is not properly believing.

Observe farther, how Christ pronounces them blessed, who should hereafter believe on him through the preaching of the gospel, though they did not see him as Thomas did, nor handle him as he might. this is a sure rule, that by how much our faith stands in less need of the external evidence of sense, the stronger our faith is, and the more acceptable it is, provided what we believe be revealed in the word of God: Blessed are they that have not seen, and yet have believed.