Cyril of Alexandria

Not firmly established is the judgment of new believers, nor is the mind firmly built upon fresh miracles. And how should they whose course of instruction was yet so to say green, be already rooted in piety? Therefore Christ doth not yet commit Himself to the novices, showing that a great thing and most worthy of love is affinity with God, and that it doth not just lie before those who desire to have it, but is achieved by zeal for good, and diligence and time. …

Commentary on the Gospel of John, Book 2 ↗

Theophylact of Ohrid

"He Himself," it says, "Jesus did not entrust Himself to them" and did not transmit the full teaching, as to those who did not truly believe, penetrating into their hearts (Ps. 93:11; Jer. 17:10) and knowing what was in them concerning Him. For it was not hidden from Him what the thought was in each person who apparently believed.

Commentary on John ↗

Bede

An admonition to us not to be confident of ourselves, but ever anxious and mistrustful; knowing that what escapes our own knowledge, cannot escape the eternal Judge.

Catena Aurea by Aquinas ↗

Thomas Aquinas

The attitude of Jesus to those who believed in him is shown when he says, But Jesus did not trust himself to them, i.e., those who had believed in him. What is this, men entrust themselves to God, and Jesus himself does not entrust himself to them? Could they kill him against his will? Some will say that he did not trust himself to them because he knew that their belief was not genuine. But if this were true, the Evangelist would surely not have said that many believed in his name, and yet he did not trust himself to them. …

Commentary on John ↗

Bonaventure

But Jesus himself did not entrust himself to them: not because he feared, but because he recognized their weakness. And he shows how he knew this: because not as a man by report, but as God by internal vision: because he himself knew all men: 1 Kings 16: "Men see those things that appear, but God beholds the heart."

Commentary on John, Chapter 2 ↗