Verse 18. For if I build again the things which I destroyed. Paul here uses the first person; but he evidently intends it as a general proposition, and means that if any one does it he becomes a transgressor. The sense is, that if a man, having removed or destroyed that which was evil, again introduces it or establishes it, he does wrong, and is a transgressor of the law of God. The particular application here, as it seems to me, is to the subject of circumcision, and the other rites of the Mosaic law. They had been virtually abolished by the coming of the Redeemer, and by the doctrine of justification by faith. It had been seen that there was no necessity for their observance, and of that Peter and the others had been fully aware. Yet they were lending their influence again to establish them, or to "build" them up again. They complied with them, and they insisted on the necessity of their observance. Their conduct, therefore, was that of building up again that which had once been destroyed -- destroyed by the ministry, and toils, and death of the Lord Jesus, and by the fair influence of his gospel. To rebuild that again, to re-establish those customs, was wrong, and now involved the guilt of a transgression of the law of God. Doddridge supposes that this is an address to the Galatians, and that the address to Peter closed at the previous verse. But it is impossible to determine this; and it seems to me more probable that this is all a part of the address to Peter, or rather, perhaps, to the assembly when Peter was present.

See Barnes "Ga 2:15".