5 Whence then does this man pray, set among evil men, with whose enmities he was being exercised? Why says he, “I have been made sad in my exercise, and have been troubled”? While he is extending his love so as to love enemies, he has been affected with disgust, being bayed at all around by the enmities of many men, by the frenzy of many and under a sort of human infirmity he has sunk. He has seen himself now begin to be pierced through with an evil suggestion of the devil, to bring on hatred against his enemies: wrestling against hatred in order to perfect love herself, in the very fight, and in the wrestling, he has been troubled.
For there is his voice in another Psalm, “My eye has been troubled, because of anger.” And what follows there? “I have waxen old among all mine enemies.” As if in storm and waves he were beginning to sink, like Peter. For he does trample the waves of this world, that loves enemies. Christ on the sea was walking fearless, from whose heart there could not by any means be taken away the love of an enemy, who hanging on the Cross did say, “Father, forgive them, for they know not what they do.” Peter too would walk.
He as Head, Peter as Body: because, “Upon this rock,” He says, “I will build My Church.” He was bidden to walk, and he was walking by the Grace of Him bidding, not by his own strength. But when he saw the wind mighty, he feared; and then he began to sink, being troubled in his exercise. By what mighty wind? “By the voice of the enemy, and by the tribulation of the sinner”. Therefore, in the same manner as he cried out on the waves, “Lord, I perish, save me,” a similar voice from this man has preceded, “Hearken unto me.”
Wherefore? For what do you suffer? Of what do you groan? “I have been made sad in my exercise.” To be exercised indeed among evil men You have set me, but too much they have risen up, beyond my powers: calm Thou one troubled, stretch forth a hand to one sinking. “For they have brought down upon me iniquity, and in anger they were shadowing me.” You have heard of waves and winds: one as it were humbled they were insulting, and he was praying: on every side against him with the roar of insult they were raging, but he within was calling upon Him whom they did not see....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)