Epiphanius of Salamis

Now the followers of Ebionand Cerinthus say, “It is enough for a disciple if he be as his Master. But Christ circumcised Himself. Be you therefore circumcised.” But herein do they deceive themselves, destroying their own principles; for if Ebion should confess that Christ as God descended from heaven and was circumcised on the eighth day, it might then afford the ground of an argument for circumcision; but since he affirms Him to be mere man, surely as a boy he cannot be the cause of Himself being circumcised, as neither are infants the authors of their own circumcision. But we confess that it is God Himself who has descended from heaven, and that enclosed in a virgin's womb, He abode there the whole time necessary for her delivery, until He should perfectly form to Himself of the virgin's womb a human body; and that in this body He was not in appearance but truly circumcised on the eighth day, in order that the figures having come to this spiritual fulfillment, both by Himself and His disciples, might now be spread abroad no longer the figures but the reality.
Christ was circumcised for several reasons. First indeed to show the reality of His flesh, in opposition to Manichaus 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, asValentinian 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.

(lib. i. Hær. 30.) Now the followers of Ebion and Cerinthus say e, "It is enough for a disciple if he be as his Master. But Christ circumcised Himself. Be thou therefore circumcised." But herein do they deceive themselves, destroying their own principles; for if Ebion should confess that Christ as God descended from heaven and was circumcised on the eighth day, it might then afford the ground of an argument for circumcision; but since he affirms Him to be mere man, surely as a boy he cannot be the cause of Himself being circumcised, as neither are infants the authors of their own circumcision. But we confess that it is God Himself who has descended from heaven, and that inclosed in a virgin's womb, He abode there the whole time necessary for her delivery, until He should perfectly form to Himself of the virgin's womb a human body; and that in this body He was not in appearance but truly circumcised on the eighth day, in order that the figures having come to this spiritual fulfilment, both by Himself and His disciples, might now be spread abroad no longer the figures but the reality.

(ubi sup.) Christ was circumcised for several reasons. First indeed to show 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.