Theophylact of Ohrid

To the disciples who asked Him privately, He says, "To you it has been given to know the mysteries" (Mt. 13:11). But surely this was not given to some and not to others by allotment and appointment of nature? It cannot be; rather it was given to them as seekers: "Seek," it says, "and it will be given to you" (cf. Mt. 7:7), while the rest God left in blindness, so that knowledge of what is right might not serve for their greater condemnation when they fail to do what is right.

Commentary on Mark ↗

Clement of Alexandria

And was it not this which the prophet meant, when he ordered unleavened cakes to be made, intimating that the truly sacred mystic word, respecting the unbegotten and His powers, ought to be concealed? In confirmation of these things, in the Epistle to the Corinthians the apostle plainly says: "Howbeit we speak wisdom among those who are perfect, but not the wisdom of this world, or of the princes of this world, that come to nought. …

The Stromata Book 5 ↗

Bede

But to those who are outside, everything is done in parables, so that seeing they may see and not perceive, and hearing they may hear and not understand, lest they turn and be forgiven their sins. It should be noted in these words of the Lord that not only what he said but also what he did were parables, that is, signs of mystical things, since it is said that those to whom everything was done in parables neither could lead what they saw and heard to understanding. …

On the Gospel of Mark ↗