A maskil once told Rav Yonason Eibeschutz zy”a that he was not going to circumcise his son. He claimed that scholars say that this practice was started by the Egyptians, who taught it to the other nations, including the Jews.
Rav Yonason told him, “I see that you like bringing proofs from non-Jewish scholars, and I agree with you. I agree that the ancient Egyptians practiced circumcision, as Rashi says that Yosef advised them to circumcise themselves. But who taught them how to do this? Yosef Hatzadik!”
The maskil said, “That doesn’t make sense. Why would Yosef tell the Egyptians to circumcise themselves? What did he care if they did this or not?”
Rav Yonason answered, “Yosef knew that there would be kofrim who liked non-Jewish sources more than Jewish ones. Therefore, he taught the Egyptians to do milah so that these heretics would want to do this mitzvah in emulation of the goyim, since they never would do so for the mitzvah.”
