Christ was circumcised for several reasons. First indeed to shew the reality of His flesh, in opposition to Manichæus f and those who say that He came forth in appearance only. Secondly, that He might prove that His body was not of the same substance with the Deity, according to Apollinaris, and that it descended not from heaven, as Valentinian said. Thirdly, to add a confirmation to circumcision which He had of old instituted to wait His coming. Lastly, to leave no excuse to the Jews. For had He not been circumcised, they might have objected that they could not receive Christ uncircumcised.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: ubi sup.