29 This is a strong pledge of the promise of God. The sons of this David are the children of the Bridegroom; all Christians therefore are called His sons. But it is much indeed that God promises, that if Christians, that is, “If his children forsake My law, and walk not in My judgments”; “if they profane My statutes, and keep not My commandments”; I will not spurn them, nor will I send them away from Me in perdition: but what will I do? “I will visit their offenses with the rod, and their sin with scourges”. It is not the mercy of one that calls them only; but also that chastises and scourges them. Let therefore your Father's hand be upon you, and if you are a good son, repel not chastening; for “what son is there, to whom his father gives not chastening?” Let Him chasten him, so long as He takes not from him His mercy: let Him beat him when obstinate, as long as He does not disinherit him. If you have well understood the promises of your Father, fear not to be scourged, but to be disinherited: “for whom the Lord loves He chastens: and scourges every son whom He receives.” Does the sinful son spurn chastening, when he sees the only Son without sin scourged? “I will visit their offenses with the rod.” Thus too the Apostle threatens: “What will you? Shall I come unto you with a rod?” Let not pious sons say, if You are coming with a rod, come not at all. For it is better to be taught with the Father's rod, than to perish in the caresses of the robber.
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.
31. “My covenant will I not profane, nor reject the thing that is gone out of my lips”. Because his sons sin, I will not on this account be found false: I have promised; I will do. Suppose they choose to sin even as past hope, and so fall into sins as to offend their Father's countenance, and deserve to be disinherited; is it not still God Himself, of whom it is said, “From these stones” He “will raise up sons to Abraham”? Therefore I tell you, brethren, many Christians sin venially, many are scourged and so corrected for their sin, chastened, and cured; many turn away altogether, striving with a stiff neck against the discipline of the Father, even wholly refusing God as their Father, though they have the mark of Christ, and so fall into such sins, that it can only be announced against them, “that they who do such things shall not inherit the kingdom of God.” Nevertheless, Christ shall not be destitute of an inheritance on their account: not for the chaff's sake shall the wheat also perish: nor on account of bad fish shall nothing be cast into the vessels from that net. “The Lord knows them that are His.” For He who predestined us before we were born, promised undoubtingly: “For whom He did predestinate, them He also called: and whom He called, them He also justified: and whom He justified, them He also glorified.” Let desperate sinners sin as far as they choose: let the members of Christ reply, “If God is with us, who shall be against us?” God will not therefore do hurt in His truth, nor will He “profane His Testament.” His Testament remains immovable, because in His foreknowledge He predestined His heirs; and “He will not reject the thing that is gone out of His lips.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)