After this, Celsus, without condemning circumcision as practised by the Jews, asserts that this usage was derived from the Egyptians; thus believing the Egyptians rather than Moses, who says that Abraham was the first among men who practised the rite. And it is not Moses alone who mentions the name of Abraham, assigning to him great intimacy with God; but many also of those who give themselves to the practice of the conjuration of evil spirits, employ in their spells the expression “God of Abraham,” pointing out by the very name the friendship (that existed) between that just man and God.
And yet, while making use of the phrase “God of Abraham,” they do not know who Abraham is! And the same remark applies to Isaac, and Jacob, and Israel; which names, although confessedly Hebrew, are frequently introduced by those Egyptians who profess to produce some wonderful result by means of their knowledge. The rite of circumcision, however, which began with Abraham, and was discontinued by Jesus, who desired that His disciples should not practise it, is not before us for explanation; for the present occasion does not lead us to speak of such things, but to make an effort to refute the charges brought against the doctrine of the Jews by Celsus, who thinks that he will be able the more easily to establish the falsity of Christianity, if, by assailing its origin in Judaism, he can show that the latter also is untrue.
Source: Against Celsus (New Advent)