12 “Deep calls unto deep with the voice of your water-spouts”. I may perhaps finish the Psalm, aided as I am by your attention, whose fervour I perceive. As for your fatigue in hearing, I am not greatly solicitous, since you see me also, who speak, toiling in the heat of these exertions. Assuredly it is from your seeing me labouring, that you labour with me: for I am labouring not for myself, but for you. “Deep calls unto deep with the voice of your water-spouts.” It was God whom he addressed, who “remembered him from the land of Jordan and Hermon.” It was in wonder and admiration he spoke this: “Abyss calls unto abyss with the voice of Your water-spouts.” What abyss is this that calls, and to what other abyss? Justly, because the “understanding” spoken of is an “abyss.” For an “abyss” is a depth that cannot be reached or comprehended; and it is principally applied to a great body of water. For there is a “depth,” a “profound,” the bottom of which cannot be reached by sounding. Furthermore, it is said in a certain passage, “Your judgments are a mighty abyss,” Scripture meaning to suggest that the judgments of God are incomprehensible. What then is the “abyss” that calls, and to what other “abyss” does it call? If by “abyss” we understand a great depth, is not man's heart, do you not suppose, “an abyss”? For what is there more profound than that “abyss”? Men may speak, may be seen by the operations of their members, may be heard speaking in conversation: but whose thought is penetrated, whose heart seen into? What he is inwardly engaged on, what he is inwardly capable of, what he is inwardly doing or what purposing, what he is inwardly wishing to happen, or not to happen, who shall comprehend? I think an “abyss” may not unreasonably be understood of man, of whom it is said elsewhere, “Man shall come to a deep heart, and God shall be exalted.” If man then is an “abyss,” in what way does “abyss” call on “abyss”? Does man “call on” man as God is called upon? No, but “calls on” is equivalent to “calls to him.” For it was said of a certain person, he calls on death; that is, lives in such a way as to be inviting death; for there is no man at all who puts up a prayer, and calls expressly on death: but men by evil-living invite death. “Deep calls on deep,” then, is, “man calls to man.” Thus is it wisdom is learned, and thus faith, when “man calls to man.” The holy preachers of God's word call on the “deep:” are they not themselves “a deep” also?...
13. “Deep calls to deep with the voice of Your water-spouts.” I, who tremble all over, when my soul was disquieted on account of myself, feared greatly on account of Your “judgments.”...Are those judgments slight ones? They are great ones, severe, hard to bear; but would they were all. “Deep calls to deep with the voice of Your water-spouts,” in that Thou threatenest, You say, that there is another condemnation in store even after those sufferings. “Deep calls on deep with the voice of Your water-spouts.” “Whither then shall I go from Your presence? And whither shall I flee from Your Spirit?” seeing that deep calls to deep, and after those sufferings severer ones are to be dreaded.
14. “All Your overhangings and Your waves have come upon me.” The “waves” in what I already feel, the “overhangings” in that You denouncest. All my sufferings are Your waves; all Your denouncements of judgments are Your “overhangings.” In the “waves” that deep “calls;” in the “overhangings” is the other “deep” which it “calls to.” In this that I suffer are all Your waves; in the severer punishment that Thou threatenest, all Your “overhangings” have come unto me. For He who threatens does not let His judgments fall upon us, but keeps them suspended over us. But inasmuch as You sit at liberty, I have thus spoken unto my soul. “Hope in God: for I will confess unto Him. My God is the saving health of my countenance.” The more numerous my sufferings, the sweeter will be Your mercy.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)