19 “Hearken unto me, O Lord, for sweet is Your mercy”. He has given this as a reason why He ought to be hearkened unto, because sweet is the mercy of God....To a man set in trouble the mercy of God must needs be sweet. Concerning this sweetness of the mercy of God see ye what in another place the Scripture says: “Like rain in drought, so beautiful is the mercy of God in trouble.” That which there he says to be “beautiful,” the same he says here to be “sweet.” Not even bread would be sweet, unless hunger had preceded. Therefore even when the Lord permits or causes us to be in any trouble, even then He is merciful: for He does not withdraw nourishment, but stirs up longing. Accordingly what says he now, “Hearken to me, O Lord, for sweet is Your mercy”? Now do not defer hearkening, in so great trouble I am, that sweet to me is Your mercy. For to this end You deferred to succour, in order that to me that wherewith You succoured might be sweet: but now no longer is there cause why You must defer; my trouble has arrived at the appointed measure of distress, let Your mercy come to do the work of goodness. “After the multitude of Your pities have regard unto me:” not after the multitude of my sins.
20. “Turn not away Your face from Your child”. And this is a commending of humility; “from Your child,” that is, “from Your little one:” because now I have been rid of pride through the discipline of tribulation, “turn not away Your face from Your child.” This is that beautiful mercy of God, whereof he spoke above. For in the following verse he explains that whereof he spoke: “For I am troubled, speedily hearken unto me.” What is “speedily”? Now there is no cause why You must defer it: I am troubled, my affliction has gone before; let Your mercy follow.
21. “Give heed to my soul, and redeem her,” does need no exposition: let us see therefore what follows. “Because of mine enemies deliver me”. This petition is evidently wonderful, neither briefly to be touched upon, nor hastily to be skipped over; truly wonderful: “Because of mine enemies deliver me.” What is, “Because of mine enemies deliver me”?...I see no reason for this petition, “Because of mine enemies deliver me:” unless we understand it of something else, which when I shall have spoken by the help of the Lord, He shall judge in you, that dwells in you. There is a kind of secret deliverance of holy men: this for their own sakes is made. There is one public and evident: this is made because of their enemies, either for their punishment, or for their deliverance. For truly God delivered not the brothers in the book of Maccabees from the fires of the persecutor....But again the Three Children openly were delivered from the furnace of fire; because their body also was rescued, their safety was public. The former were in secret crowned, the latter openly delivered: all however saved....There is then a secret deliverance, there is an open deliverance. Secret deliverance does belong to the soul, open deliverance to the body as well. For in secret the soul is delivered, openly the body. Again, if so it be, in this Psalm the voice of the Lord let us acknowledge: to the secret deliverance does belong that whereof he spoke above, “Give heed to my soul, and redeem her.” There remains the body's deliverance: for on His arising and ascending into the Heavens, and sending the Holy Ghost from above, there were converted to His faith they that at His death did rage, and out of enemies they were made friends through His grace, not through their righteousness. Therefore he has continued, “Because of mine enemies deliver me. Give heed to my soul,” but this in secret: but “because of mine enemies deliver” even my body. For mine enemies it will profit nothing if soul alone You shall have delivered; that they have done something, that they have accomplished something, they will believe. “What profit is there in my blood, while I go down into corruption?” Therefore “give heed to my soul, and redeem her,” which You alone know: secondly also, “because of mine enemies deliver me,” that my flesh may not see corruption.
22. “You know my reproach, and my confusion, and my shame”. What is reproach? What is confusion? What shame? Reproach is that which the enemy casts in the teeth. Confusion is that which gnaws the conscience. Shame is that which causes even a noble brow to blush, because of the upbraiding with a pretended crime. There is no crime; or even if there is a crime, it does not belong to him, against whom it is alleged: but yet the infirmity of the human mind ofttimes is made ashamed even when a pretended crime is alleged; not because it is alleged, but because it is believed. All these things are in the Body of the Lord. For confusion in Him could not be, in whom guilt was not found. There was alleged as a crime against Christians, the very fact that they were Christians. That indeed was glory: the brave gladly received it, and so received it as that they blushed not at all for the Lord's name. For fearlessness had covered the face of them, having the effrontery of Paul, saying, “for I blush not because of the Gospel: for the virtue of God it is for salvation to every one believing.” O Paul, are not you a venerator of the Crucified? Little it is, he says, for me not to blush for it: nay, therein alone I glory, wherefore the enemy thinks me to blush. “But from me far be it to glory, save in the Cross of Jesus Christ, through whom to me the world is crucified, and I to the world.” At such a brow as this then reproach alone could be hurled. For neither could there be confusion in a conscience already made whole, nor shame in a brow so free. But when it was being alleged against certain that they had slain Christ, deservedly they were pricked through with evil conscience, and to their health confounded and converted, so that they could say, “You have known my confusion.” You therefore, O Lord, hast known not only my reproach but also my confusion, in certain shame also: who, though in me they believe, publicly blush to confess me before ungodly men, human tongue having more influence with them than promise divine. Behold ye therefore them: even such are commended to God, not that so He may leave them, but that by aiding them He may make them perfect. For a certain man believing and wavering has said, “I believe, O Lord, help my unbelief.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)