And one of the most profound statements in that letter is:
I do not nullify the grace of God, for if righteousness were through the law, then Christ died for no purpose. (Galatians 2:21 ESV)
I ran accross this quote in one of my commentaries:
The deepest heresy of all, which corrupts churches, leavens creeds with folly, and swells our human hearts with pride, is salvation by works. "I believe, that the root of every schism and heresy from which the Christian Church has suffered, has been the effort to earn salvation rather than to receive it; and that one reason why preaching is so ineffective is that it calls on men oftener to work for God than to behold God working for them.- John Ruskin
Now, that's probably not totally true - many of the early schisms in the church, e.g. the Arian controversy, were about the nature of God and the deity of Jesus, but salvation by works is certainly a key Gospel issue, as Paul makes clear.