That the truth of the gospel. There was no danger that Paul would be deprived of his liberty even by yielding to them; but the example would have done harm to others, and therefore he prudently inquired what was expedient. This shows us how far offenses must be avoided, and points us to edification as the object which ought to be kept in view in all matters of indifference. The amount, is this: “We are the servants of the brethren, but still keeping in view that we all serve the Lord, and that the liberty of our conscience shall remain unimpaired.” When false brethren wished to bring the saints in to bondage, it was their duty not to yield to them.
The truth of the gospel denotes its genuine purity, or, which means the same thing, its pure and entire doctrine. For the false apostles did not altogether set aside the gospel, but mixed up with it their own notions, so as to give it a false and disguised aspect, which it always has when we make the smallest departure “from the simplicity that is in Christ.” (2 Cor 11:3.)
With what effrontery then will the Papists boast that they possess the gospel, which is not only corrupted by many inventions, but more than adulterated by many wicked doctrines? Let us remember that it is not enough to retain the name of the gospel, and some kind of summary of its doctrines, if its solid purity do not remain untouched. Where are the men who, by pretended moderation, endeavor to bring about a reconciliation between us and the Papists? as if the doctrine of religion, like a matter affecting money or property, could be compromised. With what abhorrence would such a transaction have been regarded by Paul, who affirms that it is not the true gospel, if it is not pure!
John Calvin, Commentary on Galatians, on Gal 2:5