Why did Paul repeat the warning about circumcision and works?
Circumcision and fleshly works, if tied to salvation, cancel the work of the cross, because God insists the work is finished. Paul testified again that anyone circumcised becomes a debtor to the whole law, which no one except Christ could keep; such legalism makes Christ of no effect and is falling from grace, not loss of salvation.
Les explains that circumcision was the big issue then because Jewish people were bringing it into Gentile churches, but he points out it can be anything done in the energy of the flesh. Joining the church, being baptized, taking the Lord's Supper, or good works—if tied to salvation—cancels the work of the cross, because God is adamant that He finished everything needed. Paul repeats the warning because the circumcised person becomes a debtor to do the whole law, and only Christ Himself ever kept the whole law, so keeping it is an utter impossibility. Verse 4 says Christ becomes of no effect to those justified by law or legalism. "Fallen from grace" does not mean saved and lost; it means they turned their back on what God provided.
For I testify again to every man that is circumcised, that he is a debtor to do the whole law.
More Questions
What kinds of religious activities could circumcision represent in Galatians 5?
Circumcision in Paul's warning can represent anything you do in the energy of the flesh, such as church membership, baptism, the Lord's Supper, or good works when those things are tied to salvation.