Why was circumcision such a big issue in the Galatian churches?
Circumcision was a major issue because Jewish people were bringing it into the Gentile churches as something tied to salvation. Paul testified that anyone circumcised for that reason became a debtor to the whole law. Such fleshly works cancel the work of the cross, make Christ of no effect, and cause a fall from grace.
Les explains that circumcision was the big deal back then because it was the Jewish people who were bringing it in among the Gentile churches. He then broadens the principle: it can be anything you do in the energy of the flesh—joining the church, being baptized, taking the Lord's Supper, or good works—and if you tie it to salvation, you cancel the work of the cross. God is adamant that He finished it and did everything that needed to be done. Paul repeats that every circumcised person becomes a debtor to do the whole law, but only Christ Himself ever kept it; keeping the whole law is an utter impossibility. So Christ becomes of no effect, and they have fallen from grace—not meaning they were saved and fell out, but that 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.