Jewish cultists wanted the Galatian Christians to undergo circumcision, in addition to their faith in Christ, with the idea that doing that would save them (see Acts 15:1). They were synthesizers. They wanted to blend Jesus Christ with keeping the Old Testament law.
Paul makes it clear. You cannot blend faith in Jesus Christ for salvation with law-keeping. It's an either/or, not a both/and. You may not combine them. You must chooses between them. Are you going to rely on Christ's death to make you right with God, or your own performance? The first option saves you, the second option condemns you.
This is why Paul says that, if they choose the circumcision route, Jesus Christ is of no benefit to them. The Galatians were also naive. It sounds as if the Jewish cultists were saying, "Just this one thing, just circumcision, that's all, then you're done." But Paul, a Benjaminite rabbi trained in the heart of Jerusalem, knows that you must keep the entire Law, down to the smallest aspects, if you decide to live by Law. You are not allowed to pick-and-choose between the laws you like and the laws you don't.
Any Christian who tries to secure his own salvation by keeping moral laws has cut himself off from Christ, and has abandoned the grace of God. The Law demands perfect righteousness. Our only hope for perfected righteousness is Christ, returning as He promised. By the Spirit's power, through faith, we eagerly wait for Him to come back.
Only faith in Christ and His gospel, working through the love that faith generates in our hearts, counts for anything. Whether you are Gentile or Jew, uncircumcised or circumcised, is irrelevant.
You must choose between salvation by Law or salvation by Christ. Which will you pick?