“So I say, walk by the Spirit, and you will not gratify the desires of the flesh.” -Galatians 5:16
They are thought to have been a tribe of Middle Eastern Gauls that made up Galatia. Immigrants from around 270 BC. Whereas Greeks had a tendency to question everything, including the obvious, these Gauls had a gift for only comprehending what they already knew. So Paul had spoken extensively to the Galatians, even lived among them, but still had to explain that the old law had been surpassed.
To be fair, the old law had not been replaced, but it is the spirit of that law, as opposed to the obvious expression of it, that the faithful follow.
Paul was trained as one who would be high Priest of the Jews. But he ended up leading the early Christians. He sets up the question of Law or faith and answers it with the examples of Hagar and Sarah. Hagar was slave to Abraham and bore him slaves. Sarah was wife to Abraham and bore him free children. According to Paul one had to follow the spirit, and by doing so, would find themselves freed of flesh demands.