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Correction: Presumably you're referring to the fact that Graham Chapman is uncircumcised and is playing a Jewish man. Brian Cohen was the illegitimate son of an anonymous Roman legionnaire and a prostitute. As his parents did not marry in the faith they would hold no kettubah - a certificate signed by a Rabbi solemnizing the Jewish marriage. Technically Brian is not Jewish, despite his claims to be a 'Red Sea pedestrian'. Besides, as the son of a prostitute with a non-Jewish father he would be an apostate, denied Jewish religious ritual such as a bar mitzvah and - more relevant to this issue - the bris, or ritual circumcision.