Previously: galatians 5:1-6, fallen from grace
You ran well. Who hindered you from obeying the truth? This persuasion does not come from Him who calls you. A little leaven leavens the whole lump. I have confidence in you, in the Lord, that you will have no other mind; but he who troubles you shall bear his judgment, whoever he is. And I, brethren, if I still preach circumcision, why do I still suffer persecution? Then the offense of the cross has ceased. I could wish that those who trouble you would even cut themselves off! Gal 5:7-12
So Paul is saying, someone came in to the church and began teaching a teaching that does not come from truth or from Him who calls us, saying that we cannot be justified unless we become circumcised and become Jews – for circumcision is the sign of the covenant between God and the Jew. God shows us even in the Torah that the circumcision of the flesh is just an outward symbol of what He really wants, which is hearts which are circumcised to Him (Deu 10:16). Being only circumcised in the flesh does not change a person’s heart. Paul is so upset about this person which has come in, that he wishes that they would emasculate themselves.
Verse 11 is key: Paul says that the cross gives offense, and it does so by providing a way (the only way) to justification which requires men to humble themselves and not receive any credit for themselves, because of their own works. So it offends the circumcision party, because they have misunderstood Moses and believe that righteous works were meant to count toward justification.
For you, brethren, have been called to liberty; only do not use liberty as an opportunity for the flesh, but through love serve one another. For all the Law is fulfilled in one word, even in this: “You shall love your neighbor as yourself.” But if you bite and devour one another, beware lest you be consumed by one another! Gal 5:13-15
Now we can put to rest the misunderstanding people have with what Paul is teaching. He says, Do not use your liberty as an opportunity for the flesh. Do not think, because what I do does not justify me or unjustify me, that I can now ignore the commands of God. I am no longer condemned by idolatry, therefore I can worship idols. I am no longer condemned by adultery, therefore I can enter into an affair; I am no longer condemned by theft, therefore I can cheat my customers.
Then Paul quotes a Law of the Torah: Love your neighbor as yourself (Lev 19:18). This Law summarizes the last five of the Ten Commandments (Love God with all your heart, soul, and strength, Deu 6:5, summarizes the first five of the Ten). So he is encouraging obedience to the Torah, but he is vehemently denying justification by our obedience to the Torah.