John 20:25

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.

Thomas sinned in this (1) by unbelief, (2) by obstinacy, (3) by pride, (4) by irreverence (for when all the other Apostles said that He had risen, he obstinately stood out, and refused to believe, (5) by presumption, because he would not believe, unless he thrust his hands into the wounds (canst thou then presume, 0 Thomas, to lay down laws for Christ?), (6) by persisting in this unbelief for eight days when, it may be, the Mother of Christ urged him to believe to be not merely unbelieving as to the mode of the resurrection (as S. Ambrose supposes), but even as to its truth, as though the other Apostles were taken in and deceived, having seen only a ghost or phantom, and not Christ Himself (See Origen, Lib. ii . Contr. Celsum; S. Augustine, Lib. xvi . Contra Faust. cap. 33; and S. Gregory, Hom. xxvi.)

Besides, this unbelief of S. Thomas' arose partly from his not believing Christ to be God. For had he believed this, he would easily have understood that Christ could have raised His Body to life again, and it is surprising that Cyril should say that Thomas believed Him to be God; and it partly arose from His excessive sorrow, especially because he alone had not seen Christ at the same time as the other Apostles. This wounded him much, and caused him to utter these bitter words. So Cyril, xii. 57. But God allowed it to be thus, in order that Thomas and we should be confirmed in humility, and in belief in the resurrection by this fresh appearance of Christ. So S. Gregory, Hom. xxvi., S. Augustine, Serm. clxi. (opus spurium), and others.

The print. In Vulgate, fixura, "the driving in" the mark which the nails made. (Pseudo)-Augustine (Serm. clix.) says, "He was seeking for the hands and the side, and while he was too curiously (dwelling on the wounds, he risked the death of his faith. The Lord wished him to see Him lest he should lose his soul by unbelief."