5 Perhaps, indeed, that circumcision pointed to the Lord Himself, at whom they were indignant, because He worked cures and healing. For circumcision was commanded to be applied on the eighth day: and what is circumcision but the spoiling of the flesh? This circumcision, then, signified the removal of carnal lusts from the heart. Therefore not without cause was it given, and ordered to be made in that member; since by that member the creature of mortal kind is procreated.
By one man came death, just as by one man the resurrection of the dead; and by one man sin entered into the world, and death by sin. Therefore every man is born with a foreskin, because every man is born with the vice of propagation; and God cleanses not, either from the vice with which we are born, or from the vices which we add thereto by ill living, except by the stony knife, the Lord Christ. For Christ was the Rock. Now they used to circumcise with stone knives, and by the name of rock they prefigured Christ; and yet when He was present with them they did not acknowledge Him, but besides, they sought to kill Him.
But why on the eighth day, unless because after the seventh day of the week the Lord rose again on the Lord's day? Therefore Christ's resurrection, which happened on the third day indeed of His passion, but on the eighth day in the days of the week, that same resurrection it is that does circumcise us. Hear of those that were circumcised with the real stone, while the apostle admonishes them: “If then ye be risen with Christ, seek those things which are above, where Christ is, sitting on the right hand of God; set your affection on things above, not on things on the earth.” He speaks to the circumcised: Christ has risen; He has taken away from you carnal desires, evil lusts, the superfluity with which you were born, and that far worse which you had added thereto by ill living; being circumcised by the Rock, why do you still set your affections on the earth?
And finally, for that “Moses gave you the law, and you circumcise a man on the Sabbath day,” understand that by this is signified the good work which I have done, in that I have made a man every whit whole on the Sabbath day; because he was cured that he might be whole in body, and also he believed that he might be whole in soul.
Source: Tractates on the Gospel of John (New Advent)