Augustine of Hippo

"But his delight is in the law of the Lord, and in His law will he meditate by day and by night (ver. 2). The law is not made for a righteous man," says the Apostle. But it is one thing to be in the law, another under the law. Whoso is in the law, acteth according to the law; whoso is under the law, is acted upon according to the law: the one therefore is free, the other a slave. …

On the Psalms, Psalm 1 ↗

Ambrose of Milan

"But in the law of the Lord was his will, and in his law he shall meditate day and night"; that is, blessed is he who does these things with counsel, reason, prudence; for even a small child can abstain from those things which have been said, not by virtue, but by impossibility and ignorance of sinning. Even an irrational beast can comply, to which there is no power of counsel, no sense of error. …

Commentaries on the Twelve Davidic Psalms ↗

Jerome

And in his law he shall meditate day and night. He named three things which he ought not to do: not to walk in the counsel of the ungodly, and not to stand in the way of sinners, and not to sit in the seat of pestilence. He named three things which we ought not to do: he names two things which we ought to do. For it is not enough for us to flee evils, unless we attain to goods. But his will is in the law of the Lord. He did not say "fear," but "will": for many act out of fear, but the fear of the one acting has no reward. …

Homily 1, on Psalm 1 ↗