Circumcision of an infant meant that the infant was, as a result, a member of the local synagogue. So if there was Christian circumcision (and there was), and if there were Christian synagogues (and there were), and if the Christians who went to these synagogues were the same believers who circumcised their sons (and they were), then the necessary conclusion is that we know with certainty that some first century Christian churches had infant members.
