Augustine of Hippo

"For if an enemy had upbraided me" [Psalm 55:12]. And indeed above he was "troubled in his exercise" by the voice of the enemy and by the tribulation of the sinner, perhaps being placed in that city, that proud city that was building a tower, which was "sunk," that divided might be the tongues: give heed to his inward groaning because of perils from false brethren. …

Exposition on Psalm 55 ↗

Ambrose of Milan

So one who does the will of God is his friend and is honored with this name. He who is of one mind with him, he too is his friend. For there is unity of mind in friends, and no one is more hateful than the person who injures friendship. Hence in the traitor the Lord found this the worst point on which to condemn his treachery, namely, that Judas gave no sign of gratitude and had mingled the poison of malice at the table of friendship. …

On the Duties of the Clergy 3.136 ↗

Thomas Aquinas

"If my enemy." Here he describes the malice of a certain singular and principal person in the multitude. And perhaps it refers to Saul, or to Doeg the Edomite. And first he sets forth an evil that could be tolerated in some way. Second, he sets forth an evil that is intolerable. It is tolerable in some way that a man should suffer persecution from enemies. …

Exposition on the Psalms of David ↗