In these words, “I am crucified with Christ,” he alludes to Baptism and in the words “nevertheless I live, yet not I,” our subsequent manner of life whereby our members are mortified. By saying “Christ lives in me,” he means nothing is done by me, which Christ disapproves; for as by death he signifies not what is commonly understood, but a death to sin; so by life, he signifies a delivery from sin. For a man cannot live to God, otherwise than by dying to sin; and as Christ suffered bodily death, so does Paul a death to sin. “Mortify,” says he, “your members which are upon the earth; fornication, uncleanness, passion;”, and again, “our old man was crucified,” which took place in the Bath. After which, if you remain dead to sin, you live to God, but if you let it live again, you are the ruin of your new life. This however did not Paul, but continued wholly dead; if then, he says, I live to God a life other than that in the Law, and am dead to the Law, I cannot possibly keep any part of the Law. Consider how perfect was his walk, and you will be transported with admiration of this blessed soul. He says not, “I live,” but, “Christ lives in me;” who is bold enough to utter such words? Paul indeed, who had harnessed himself to Christ's yoke, and cast away all worldly things, and was paying universal obedience to His will, says not, “I live to Christ,” but what is far higher, “Christ lives in me.” As sin, when it has the mastery, is itself the vital principle, and leads the soul whither it will, so, when it is slain and the will of Christ obeyed, this life is no longer earthly, but Christ lives, that is, works, has mastery within us. His saying, “I am crucified with Him” “I no longer live,” but “am dead,” seeming incredible to many, he adds,
Ver. 20. “And that life which I now live in the flesh, I live in faith, the faith which is in the Son of God.”
The foregoing, says he, relates to our spiritual life, but this life of sense too, if considered, will be found owing to my faith in Christ. For as regards the former Dispensation and Law, I had incurred the severest punishment, and had long ago perished, “for all have sinned, and come short of the glory of God.” And we, who lay under sentence, have been liberated by Christ, for all of us are dead, if not in fact, at least by sentence; and He has delivered us from the expected blow. When the Law had accused, and God condemned us, Christ came, and by giving Himself up to death, rescued us all from death. So that “the life which I now live in the flesh, I live in faith.” Had not this been, nothing could have averted a destruction as general as that which took place at the flood, but His advent arrested the wrath of God, and caused us to live by faith. That such is his meaning appears from what follows. After saying, that “the life which I now live in the flesh, I live in faith,” he adds,
Ver. 20. “In the Son of God, Who loved me, and gave Himself up for me.”
How is this, O Paul! Why do you appropriate a general benefit, and make your own what was done for the whole world's sake? For he says not, “Who loved us,” but, “Who loved me.” And besides the Evangelist says, “God so loved the world;” and Paul himself, “He that spared not His own Son, but delivered Him up,” not for Paul only, but, “for us all;” and again, “that He might purify unto himself a people for his own possession,” But considering the desperate condition of human nature, and the ineffably tender solicitude of Christ, in what He delivered us from, and what He freely gave us, and kindled by the yearning of affection towards Him, he thus expresses himself. Thus the Prophets often appropriate to themselves Him who is God of all, as in the words, “O God, you are my God, early will I seek You.” Moreover, this language teaches that each individual justly owes as a great debt of gratitude to Christ, as if He had come for his sake alone, for He would not have grudged this His condescension though but for one, so that the measure of His love to each is as great as to the whole world. Truly the Sacrifice was offered for all mankind, and was sufficient to save all, but those who enjoy the blessing are the believing only. Nevertheless it did not deter Him from His so great condescension, that not all would come; but He acted after the pattern of the supper in the Gospel, which He prepared for all, yet when the guests came not, instead of withdrawing the viands, He called in others. So too He did not despise that sheep, though one only, which had strayed from the ninety and nine. This too in like manner St. Paul intimates, when he says, speaking about the Jews, “For what if some were without faith, shall their want of faith make of none effect the faithfulness of God? God forbid: yea let God be found true, but every man a liar.” When He so loved you as to give Himself up to bring you who wast without hope to a life so great and blessed, can you, thus gifted, have recourse to things gone by? His reasoning being completed, he concludes with a vehement asseveration, saying,
Ver. 21. “I do not make void the grace of God.”
Let those, who even now Judaize and adhere to the Law, listen to this, for it applies to them.
Ver. 21. “For if righteousness is through the Law, then Christ died for naught.”
What can be more heinous than this sin? what more fit to put one to shame than these words? Christ's death is a plain proof of the inability of the Law to justify us; and if it does justify, then is His death superfluous. Yet how could it be reasonable to say that has been done heedlessly and in vain which is so awful, so surpassing human reason, a mystery so ineffable, with which Patriarchs travailed, which Prophets foretold, which Angels gazed on with consternation, which all men confess as the summit of the Divine tenderness? Reflecting how utterly out of place it would be if they should say that so great and high a deed had been done superfluously, (for this is what their conduct came to,) he even uses violent language against them, as we find in the words which follow.
Source: Commentary on Galatians (New Advent)