The reason for the criticism of the circumcised believers is given in Acts 11:3. They are upset that Peter went to eat with a Gentile. Part of their opposition may be the fact that Jews will persecute Christians if they find out that Christians do not think it is important or necessary to continue abiding by the food laws. If Christians are known to associate with Gentiles where those Gentiles are not circumcised, this would bring shame upon them in the Jewish communities where they lived.1
2 So when Peter went up to Jerusalem, the circumcision party criticized him, saying,