Someone, or even a group of people, traveled among the churches of Galatia, teaching that members of the church had to be circumcised according to the Law of Moses to be saved. Paul wrote Galatians to refute that teaching.
He argued what we have to have to be as Abraham. He had faith before his circumcision and by that he found justification before God.
Likewise, Paul wrote,
“Therefore know that only those who are of faith are sons of Abraham” (Galatians 3:7).
Live by faith, not by the works of the Law, and you shall be what Abraham was, the friend of God.