Augustine of Hippo

Therefore, He governs you, who carries you. For when you say, The Lord is my shepherd; there is nothing of your own to trust in yourself. Therefore, be careful not to boast of your merits. For there were none, when the Lord came to lift you up. He found you naked, not clothed; wounded, not healthy; lying down, not standing; wandering, not returning. Beware of arrogance, beware; for He who mercifully lifted you from the earth half-dead, carries the humble, but hurls down the proud. …

Sermon 366:2 ↗

Ambrose of Milan

How often have you heard Psalm 23 and not understood it! See how it is applicable to the heavenly sacraments: "The Lord feeds me, and I shall want nothing; he has set me in a place of pasture; he has brought me on the water of refreshment; he has converted my soul. He has led me on the paths of justice for his own name's sake. …

On the Sacraments, Book 5, Chapter 3 ↗

Thomas Aquinas

Above, the Psalmist, speaking in the person of Christ, spoke much of his tribulation; here he speaks of the remedy by which he is sustained in it. And first he commemorates the benefits by which he is sustained. Second, the power of that sustaining, at "the earth is the Lord's." This Psalm does not have a new title, but seems to designate certain persons walking on a journey. Hence it can signify the return of the people from Babylon, and it signifies the return of Christ from the world to heaven. Concerning this he does two things. …

Exposition on the Psalms of David ↗