"Some men came down from Judea to Antioch and were teaching the brothers: 'Unless you are circumcised, according to the custom taught by Moses, you cannot be saved'" (v. 1).
While Barnabas was commissioned by the Jerusalem church and sent to Antioch, some other men took it upon themselves to go to Antioch and bring their opinions to that church. They taught that all Gentile converts had to be circumcised, according to the "custom of Moses." They could not refer to the law of Moses, since God's law never required that Gentile proselytes be circumcised. Evidently these men reasoned that if Jew and Gentile were now going to be on an equal footing in the new covenant church, both equally members of the "inner circle," this meant Gentiles needed to be circumcised.
The apostles maintained a different position. They held that the whole Jew-Gentile distinction in the old covenant had been abolished in essence by the work of Jesus. The new covenant did not mean that Gentiles now became Jews; rather, it meant that believing Jew and believing Gentile both became Christians together. Thus, Peter did not circumcise Cornelius but baptized him into the new body of the church.
Moreover, circumcision was a bloody rite, and thus sacrificial. All such sacrificial rites were fulfilled at the Cross, and have no place afterwards. To practice circumcision (not for medical but for religious reasons) shows a failure to understand the work of Christ.
Because of the teachings of the "Judaizers" at Antioch, Paul and Barnabas went to Jerusalem to meet with the apostles and elders to formulate a ruling on this matter. After discussion, Peter addressed the council and reminded them that God had baptized Cornelius with the Holy Spirit, "just as He did to us" (Acts 15:8). This, according to Peter, showed that God "made no distinction between us and them" (v. 9). The old covenant distinctions were abolished not by circumcising Gentiles but by baptizing both Jew and Gentile.
The council decided that Peter and Paul were right, and the Judaizers were wrong. Sadly, the Judaizers did not submit to the apostles but formed their own sect and began troubling the church everywhere. They dogged Paul's heels and sought to impose their Jewish legalisms on the churches to which Paul ministered.
Coram Deo
The church is not immune to the temptation to elevate "custom" to the level of law. Fueling that temptation is the desire to see those who are different become like us. Examine the standard with which you judge people. Does it focus on conformity to custom, to others, or to the very Word of God?