Theophylact of Ohrid

Because he believed Mary and did not think of the resurrection? Because they did not yet know "the Scripture, that He must rise from the dead," and they believed Mary, who suspected that the body had been stolen and moved.

Commentary on John ↗

Gregory the Dialogist

"For they did not yet know the Scriptures, that he must rise from the dead." In this matter the greatness of divine providence must be considered, that the hearts of the disciples are both kindled to seek and yet delayed from finding, so that the weakness of the soul, tormented by its own sorrow, might become purer for finding, and might hold on more firmly when it found, the later it found what it was seeking.

Forty Gospel Homilies, Homily 22 ↗

Thomas Aquinas

2488 Next when he says, he saw and believed, we see the effect of the investigation. At first glance it seems to mean that he saw the situation and believed that Christ had arisen. But according to Augustine this is not correct, because the next thing the Evangelist says is, for as yet they did not know the scripture, that he must rise from the dead. Therefore, one must say that he saw the empty tomb and believed what the woman had said, which is that someone had taken the Lord. …

Commentary on John ↗

Bonaventure

For he did not yet know the Scripture, that He must rise from the dead: therefore he did not believe that He had risen, but that He had been taken away. Nevertheless, if we wish to hold the exposition of Chrysostom, the following text is continued thus: He saw and believed, that is, then for the first time he believed from seeing, because before he did not believe; for he did not yet know the Scripture.

Commentary on John, Chapter 20 ↗