Although the Jews, then, pride themselves on circumcision, they will separate it not only from that of the Colchians and Egyptians, but also from that of the Arabian Ishmaelites; and yet the latter was derived from their ancestor Abraham, the father of Ishmael, who underwent the rite of circumcision along with his father. The Jews say that the circumcision performed on the eighth day is the principal circumcision, and that which is performed according to circumstances is different; and probably it was performed on account of the hostility of some angel towards the Jewish nation, who had the power to injure such of them as were not circumcised, but was powerless against those who had undergone the rite.
This may be said to appear from what is written in the book of Exodus, where the angel before the circumcision of Eliezer was able to work against Moses, but could do nothing after his son was circumcised. And when Zipporah had learned this, she took a pebble and circumcised her child, and is recorded, according to the reading of the common copies, to have said, “The blood of my child's circumcision is stayed,” but according to the Hebrew text, “A bloody husband are you to me.” For she had known the story about a certain angel having power before the shedding of the blood, but who became powerless through the blood of circumcision.
For which reason the words were addressed to Moses, “A bloody husband are you to me.” But these things, which appear rather of a curious nature, and not level to the comprehension of the multitude, I have ventured to treat at such length; and now I shall only add, as becomes a Christian, one thing more, and shall then pass on to what follows. For this angel might have had power, I think, over those of the people who were not circumcised, and generally over all who worshipped only the Creator; and this power lasted so long as Jesus had not assumed a human body.
But when He had done this, and had undergone the rite of circumcision in His own person, all the power of the angel over those who practise the same worship, but are not circumcised, was abolished; for Jesus reduced it to nought by (the power of) His unspeakable divinity. And therefore His disciples are forbidden to circumcise themselves, and are reminded (by the apostle): “If you be circumcised, Christ shall profit you nothing.”
Source: Against Celsus (New Advent)