Galatians 2:15-16 (ESV)
We ourselves are Jews by birth and not Gentile sinners;  yet we know that a person is not justified by works of the law but through faith in Jesus Christ, so we also have believed in Christ Jesus, in order to be justified by faith in Christ and not by works of the law, because by works of the law no one will be justified.
Paul continues his discourse with Peter. “We know that a person is not justified by works of the law, but through faith in Jesus Christ.” This is the gospel! Notice this applies even after one becomes a Christian. We are not justified by works of the law, but through faith in Jesus Christ. Peter had been a Christian for quite sometime, as well as had Paul. Paul doesn’t send people back to the law. No, he rebukes them, Peter included, for returning to that vomit! Justification by faith alone is central to the life of a Christian, even the mature Christian. There is never a time this side of glory that we will ever be ready to move beyond this. Our lives as Christians survive by this alone. We are justified through faith in Christ. We are justified by faith in Christ. Not by works of the law. And what are these works of the law that Peter is guilty of? Yes he is guilty of them! He stands accused and condemned by them! The works of the law have become sinful. Peter was actually sinning by following these laws! What were they? They were the kosher laws, from all I can tell. Peter sinned by not eating pork. Amazing. Not that you have to eat pork, but if you think you are going to save yourself by not doing something, then your fast becomes a sin. We are justified in Christ, not by works of the law. When we try to justify ourselves by the law, we sin. When we let others make rules to follow for salvation and don’t speak out, we sin. Of course, we sin in all we do, which is why we need to be justified by faith in Christ alone. Now justified in Christ, we can do all things to the glory of God, even eat pork, drink coffee, or, heaven forbid, even a beer.