How do we know that believers are the circumcision of the heart?

The Apostle Paul clarifies that true circumcision is that which occurs inwardly and is of the heart, as expressed in Romans 2:29. This spiritual circumcision is distinguished from that of the flesh and indicates that true believers, whom Paul refers to as the circumcision, are those who worship God in spirit. This circumcision of the heart involves transformation and is made possible by the Holy Spirit, who brings faith and enables believers to respond to God. This doctrinal understanding underscores the radical change that faith in Christ brings to the believer's life.