Irenaeus

They maintain that Daniel also set forth the same thing when he begged of the angels explanations of the parables, as being himself ignorant of them. But the angel, hiding from him the great mystery of Bythus, said unto him, "Go thy way quickly, Daniel, for these sayings are closed up until those who have understanding do understand them, and those who are white be made white." Moreover, they vaunt themselves as being the white and the men of good understanding.

Against Heresies Book I ↗

Jerome

"And I heard, and understood not. And I said, 'O my lord, what shall happen after these things?' And he said, 'Go, Daniel, for the words are shut up and sealed until the time of the end. Many shall be chosen and made white and shall be tried as fire; and the wicked shall deal wickedly. And none of the wicked shall understand, but the learned shall understand.'" The prophet wished to comprehend what he had seen, or rather, what he had heard, and he desired to understand the reality of the things to come. …

Commentary on Daniel ↗

Bonaventure

Piety is profitable for knowing truths. God gives His gifts upon every creature: but the knowledge of truth He gives only to those who act piously. It is said in Daniel: "Moreover the impious shall act impiously": revelation shall be closed to the impious. If you wish to be true scholars, it is necessary that you have piety.

Collationes de Septem Donis, Collation 3 ↗