I do not reject. There is great emphasis in this expression; for how dreadful is the ingratitude manifested in despising the grace of God, so invaluable in itself, and obtained at such a price! Yet this heinous offense is charged against the false apostles, who were not satisfied with having Christ alone, but introduced some other aids towards obtaining salvation. For, if we do not renounce all other hopes, and embrace Christ alone, we reject the grace of God. And what resource is left to the man, who "puts from him" the grace of God, "and judges himself unworthy of everlasting life?" (Acts 13:46.) Christ is dead in vain [1] There would then have been no value in the death of Christ; or, Christ would have died without any reward; for the reward of his death is, that he has reconciled us to the Father by making an atonement for our sins. Hence it follows, that we are justified by his grace, and, therefore, not by works. The Papists explain this in reference to the ceremonial law; but who does not see that it applies to the whole law? If we could produce a righteousness of our own, then Christ has suffered in vain; for the intention of his sufferings was to procure it for us, and what need was there that a work which we could accomplish for ourselves should be obtained from another? If the death of Christ be our redemption, then we were captives; if it be satisfaction, we were debtors; if it be atonement, we were guilty; if it be cleansing, we were unclean. On the contrary, he who ascribes to works his sanctification, pardon, atonement, righteousness, or deliverance, makes void the death of Christ. This argument, we shall perhaps be told, is of no weight against those who propose to unite the grace of Christ with works; which, it is universally admitted, was done by the false apostles. The two doctrines, it is alleged, stand together, that righteousness is by the law, and that we are redeemed by the death of Christ. True; supposing it were granted that a part of our righteousness is obtained by works, and a part comes from grace. But such theology, it may easily be proved, was unknown to Paul. His argument with his opponents is either conclusive or inconclusive. If any blasphemer shall dare to accuse him of bad reasoning, a powerful defense is at hand; for that justification in the sight of God of which he treats, is not what men may imagine to be sufficient, but what is absolutely perfect. But we are not now called to plead in behalf of Paul against blasphemers, who venture to speak in reproachful language of the Holy Spirit himself. Our present business is with the Papists. They ridicule us, when we argue with Paul that, if righteousness come by works, Christ is dead in vain. They imagine it to be a beautiful reply, with which their sophists furnish them, that Christ merited for us the first grace, that is, the opportunity of meriting; and that the merit of his death concurs with the satisfactions of works for the daily pardon of sins. Let them ridicule Paul, whose language we quote. They must refute him before they can refute us. We know that he had to deal with men, who did not entirely reject the grace of Christ, but ascribed the half of salvation to works. In opposition to them he argues, that "if righteousness is by the law, then Christ is dead in vain;" and by so doing, he certainly does not allow to works one drop of righteousness. Between those men and the Papists there is no difference; and therefore, in refuting them, we are at liberty to employ Paul's argument.

Footnotes

1 - "Dorean apethane does not mean in vain,' uselessly,' ineffectually,' but without just cause;' for if righteousness be by the law, there was no reason why he should die." -- Tittmann. Ei gar apethanen ho Christos eudelon hoti dia to me ischuein ton nomon hemas dikaioun; ei d ' ho nomos dikaioi perittos ho tou Christou thanatos. "For if Christ died, it is very evident that it was because the law was unable to justify us; and if the law justifies us, the death of Christ was superfluous." -- Chrysostom.

I do not frustrate the grace of God - The word rendered "frustrate" (ἀθετῶ athetō) means properly to displace, abrogate, abolish; then to make void, to render null; Mark 7:9; Luke 7:30; 1-Corinthians 1:19. The phrase "the grace of God," here refers to the favor of God manifested in the plan of salvation by the gospel, and is another name for the gospel. The sense is, that Paul would not take any measures or pursue any course that would render that vain or inefficacious. Neither by his own life, by a course of conduct which would show that it had no influence over the heart and conduct, nor by the observance of Jewish rites and customs, would he do anything to render that inefficacious. The design is to show that he regarded it as a great principle that the gospel was efficacious in renewing and saving man, and he would do nothing that would tend to prevent that impression on mankind. A life of sin, of open depravity and licentiousness, would do that. And in like manner a conformity to the rites of Moses as a ground of justification would tend to frustrate the grace of God, or to render the method of salvation solely by the Redeemer nugatory. This is to be regarded, therefore as at the same time a reproof of Peter for complying with customs which tended to frustrate the plan of the gospel, and a declaration that he intended that his own course of life should be such as to confirm the plan, and show its efficacy in pardoning the sinner and rendering him alive in the service of God.
For if righteousness come by the law - If justification can be secured by the observance of any law - ceremonial or moral - then there was no need of the death of Christ as an atonement. This is plain. If man by conformity to any law could be justified before God, what need was there of an atonement? The work would then have been wholly in his own power, and the merit would have been his. It follows from this, that man cannot be justified by his own morality, or his alms-deeds, or his forms of religion, or his honesty and integrity. If he can, he needs no Saviour; he can save himself. It follows also that when people depend on their own amiableness, and morality, and good works, they would feel no need of a Saviour; and this is the true reason why the mass of people reject the Lord Jesus. They suppose they do not deserve to be sent to hell. They have no deep sense of guilt. They confide in their own integrity, and feel that God ought to save them. Hence, they feel no need of a Saviour; for why should a person in health employ a physician? And confiding in their own righteousness, they reject the grace of God, and despise the plan of justification through the Redeemer. To feel the need of a Saviour it is necessary to feel that we are lost and ruined sinners; that we have no merit upon which we can rely; and that we are entirely dependent on the mercy of God for salvation. Thus feeling, we shall receive the salvation of the gospel with thankfulness and joy, and show that in regard to us Christ is not "dead in vain."

I do not frustrate - Ουκ αθετω· I do not contemn, despise, or render useless, the grace of God - the doctrine of Christ crucified; which I must do if I preach the necessity of observing the law.
For if righteousness - If justification and salvation come by an observance of the law, then Christ is dead in vain; his death is useless if an observance of the law can save us; but no observance of the law can save us, and therefore there was an absolute necessity for the death of Christ.
1. The account of the prevarication of Peter in the preceding chapter teaches us a most useful lesson. Let him who assuredly standeth take heed lest he fall. No person in a state of probation is infallible; a man may fall into sin every moment; and he will, if he do not walk with God. Worldly prudence and fleshly wisdom would have concealed this account of the prevarication of Peter; but God tells truth. This the fountain of it; and from him we are to expect not only nothing but the truth, but also the whole truth. If the Gospel were not of God we had never heard of the denial and prevarication of Peter, nor of the contention between Paul and Barnabas. And these accounts are recorded, not that men may justify or excuse their own delinquencies by them, but that they may avoid them; for he must be inexcusable who, with these histories before his eyes, ever denies his Master, or acts the part of a hypocrite. Had the apostles acted in concert to impose a forgery on the world as a Divine revelation, the imposture would have now come out. The falling out of the parties would have led to a discovery of the cheat. This relation, therefore, is an additional evidence of the truth of the Gospel.
2. On, I through the law am dead to the law, etc., pious Quesnel makes the following useful reflections:
"The ceremonial law, which is no more than a type and shadow of him, destroys itself by showing us Jesus Christ, who is the truth and the substance. The moral law, by leaving us under our own inability under sin and the curse, makes us perceive the necessity of the law of the heart, and of a Savior to give it. The law is for the old man, as to its terrible and servile part; and it was crucified and died with Christ upon the cross as well as the old man. The new man, and the new law, require a new sacrifice. What need has he of other sacrifices who has Jesus Christ? They in whom this sacrifice lives, do themselves live to God alone; but none can live to him except by faith; and this life of faith consists in dying with Christ to the things of the present world, and in expecting, as co-heirs with him, the blessings of the eternal world. And who can work all this in us but only he who lives in us? That man has arrived to a high degree of mortification, who can say Christ liveth in me, and I am crucified to the world. Such a one must have renounced not only earthly things, but his own self also."
3. Is there, or can there be, any well grounded hope of eternal life but what comes through the Gospel? In vain has the ingenuity of man tortured itself for more than 5000 years, to find out some method of mending the human heart: none has been discovered that even promised any thing likely to be effectual. The Gospel of Christ not only mends but completely cures and new makes infected nature. Who is duly apprised of the infinite excellency and importance of the Gospel? What was the world before its appearance? What would it be were this light extinguished? Blessed Lord! let neither infidelity nor false doctrine rise up to obscure this heavenly splendor!

(5) I do not frustrate the grace of God: for if righteousness [come] by the law, then Christ is dead (e) in vain.
(5) The second argument taken from an absurdity: if men may be justified by the Law, then it was not necessary for Christ to die.
(e) For there was no reason why he should do so.

I do not frustrate the grace of God,.... Or "cast it away", as the Vulgate Latin version reads it; or "deny it", as the Syriac and Arabic; or "despise, reject, and make it void", as other versions; meaning either the grace of the Son of God in giving himself for him, just mentioned by him; or the particular doctrine of grace, justification, he is speaking of, as proceeding from the grace of God, upon the foot of the righteousness of Christ; or the whole Gospel, all and each of which would be denied, despised, rejected, made null and void, be in vain, fallen and departed from, should justification be sought for by the works of the law: but this the apostle did not do, and therefore did not frustrate the grace of God: which to do would be to act the most ungenerous and ungrateful part to God, and Christ, and to that love and grace which are so largely displayed in the free justification of a sinner.
For if righteousness come by the law; if a justifying righteousness is to be attained unto by the works of the law, or men can be justified by their obedience to it,
then Christ is dead in vain; there was no necessity for his dying: he died without any true reason, or just cause; he died to bring in a righteousness which might have been brought in without his death, and so his blood and life might have been spared, his sufferings and death being entirely unnecessary; which to say is to cast contempt upon the wisdom, love, and grace of God in this matter, and to offer the greatest indignity to the person, character, sufferings, and death of Christ. Wherefore it may be strongly concluded, that there is no righteousness by the law of works, nor to be attained that way, otherwise Christ had never died; and that justification is solely and alone by his righteousness.

I do not frustrate the grace of God--I do not make it void, as thou, Peter, art doing by Judaizing.
for--justifying the strong expression "frustrate," or "make void."
is dead in vain--Greek, "Christ died needlessly," or "without just cause." Christ's having died, shows that the law has no power to justify us; for if the law can justify or make us righteous, the death of Christ is superfluous [CHRYSOSTOM].

Meantime I do not make void - In seeking to be justified by my own works. The grace of God - The free love of God in Christ Jesus. But they do, who seek justification by the law. For if righteousness is by the law - If men might be justified by their obedience to the law, moral or ceremonial. Then Christ died in vain - Without any necessity for it, since men might have been saved without his death; might by their own obedience have been both discharged from condemnation, and entitled to eternal life.