the jews maintained the practise following the israelite tribes' enslavement by the egyptian empire (c.1400-1300 BC). the ancient egyptians and abyssinians routinely circumcized slaves, both as dehumanizing procedure and as a means of ready identification. for some peculiar reason, probably simple tribal identity, the israelites continued to perform circumcisions on their children even after leaving egypt. the "god told us to do it" nonsense is - as it always is - a fallacious justification by religious authorities, invented long after the fact.