30 “Nevertheless, My mercy will I not utterly take from Him”. From whom? From that David to whom I gave these promises, whom “I anointed with my holy oil of gladness above His fellows.” Do you recognise Him from whom God will not utterly take away His mercy? That no one may anxiously say, since He speaks of Christ as Him from whom He will not take away His mercy, What then will become of the sinner? Did He say anything like this, “I will not take My loving-kindness utterly from them”?
“I will visit,” He says, “their offenses with the rod, and their sin with scourges.” You expected for your own security, “I will not utterly take my loving-kindness from” them. And indeed this is the reading of some books, but not of the most accurate: though, where they have it, it is a reading by no means inconsistent with the real meaning. For how can it be said that He will not utterly take His mercy from Christ? Has the Saviour of the body committed anything of sin either in Heaven or in earth, “who sits even at the right hand of God, who also makes intercession for us”? Yet it is from Christ: but from His members, His body which is the Church.
For in this sense He speaks of it as a great thing that He will not take away His mercies from Him, supposing us not to recognise the only Son, who is in the bosom of the Father; for there the Man is not counted for His Person, but the One Person is God and Man. He therefore does not utterly take His mercies from Him, when He takes not His mercy from His body, His members, in which, even while He was enthroned in Heaven, He was still suffering persecutions on earth; and when He cried from Heaven, “Saul, Saul,” not why do you persecute My servants, nor why do you persecute My saints, nor My disciples, but, “why do you persecute Me?” As then, while no one persecuted Him when sitting in Heaven, He cried out, “Why do you persecute Me?” when the Head recognised its limbs, and His love allowed not the Head to separate Himself from the union of the body: so, when He takes not away His mercies from Him, it is surely that He takes it not from us, who are His limbs and body.
Yet ought we not on that account to sin not without apprehension, and perversely to assure ourselves that we shall not perish, be our actions what they may. For there are certain sins and certain offenses, to define and discourse of which it is either impossible for me, or if it were possible, it would be too tedious for the time we have at present. For no man can say that he is without sin; for if he says so, he will lie; “if we say that we have no sin, we deceive ourselves, and the truth is not in us.” Each one therefore is needfully scourged for his own sins; but the mercy of God is not taken away from him, if he be a Christian.
Certainly if you commit such offenses as to repel the hand of Him who chastens, the rod of Him who scourges you, and art angry at the correction of God, and fliest from your Father when He chastens you, and will not suffer Him to be your Father, because He spares you not when thou dost sin; you have estranged yourself from your heritage, He has not thrown you off; for if you would abide being scourged, you would not abide disinherited. “Nor will I do hurt in My truth.” For His mercy in setting free shall not be taken away, lest His truth in taking vengeance do harm.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)