Verse 3:3 explains the safety Paul had referred to in verse 1. Paul promoted salvation by faith, not by following Jewish customs. As the most obvious physical sign of Judaism, "circumcision" is often used as a metaphor for the entire Old Testament law. Instead of hollow legalism, faithful followers of Jesus are marked by three practices. First, their worship is spiritual and recognizable by focusing on the Holy Spirit's work in their lives. Second, they put their faith and worship in Christ, not the law.
The Old Testament law looked forward to the Messiah but did not know who He would be. Believers glory in Christ Jesus, revealed as the Son of God, the predicted Jewish Messiah. Third, Christians don't stake their eternity or spiritual lives on rituals or good works. Again, circumcision is part of the general context here. A Christian's proper confidence is not in whether a person has been circumcised or followed some other religious ritual but in whether they have received salvation through faith in Jesus.