John Chrysostom

"But strong meat belongeth to them that are of full age, even them who by reason of use have their senses exercised to discern both good and evil." Those had not "their senses exercised," nor did they "know good and evil." He is not speaking now concerning life and conduct, when he says "to discern good and evil," for this is possible and easy for every man to know, but concerning doctrines that are wholesome and sublime, and those that are corrupted and low. The babe knows not how to distinguish bad and good food. …

Homily on Hebrews 8 ↗

Theophylact of Ohrid

That is, the lofty dogmas concerning the Divinity of Christ. Do you see that there is another infancy—that of mind—which even the elderly possess; and there is a perfection which nothing prevents even the young from having. By "habit" he means the perfection and firmness of the disposition of the moral life. …

Commentary on Hebrews ↗

Oecumenius

And he speaks of the word of righteousness, which concerns the divinity of the Lord. "solid food." The lofty word, and the high doctrines concerning Christ. "by constant practice." He refers to habit, perfection, the well-trained, having their senses exercised, such as hearing, tongue, eyes. Indeed, he speaks of the senses of the soul. To distinguish, he says, what is good, and high doctrines, and what are the spurious and corrupt. He hints at those who teach anything to everyone, giving themselves and being persuaded. …

The Pseudo-Oecumenian Catena on Hebrews ↗

Thomas Aquinas

273. – Then when he says, but solid food is for the mature, he explains what he had said about solid food. For this is evident in bodily food: when a man reaches maturity, he uses stronger and nobler and more solid food. But a spiritual man, when he has reached spiritual perfection, should have a more solid doctrine proposed to him. …

Commentary on Hebrews ↗