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.