Genesis 17:11

Abraham is ordered to circumcise himself when he is about to receive the inheritance of a true progeny. Is it not evident that "circumcision of the flesh" is the precept of chastity, that one should remove the passions of the flesh and curb the desires that unbridled lust renders indomitable? Indeed, the very word circumcision prescribes this, that every stench of impurity be wiped away and that the stimulus of the passions be removed.

WHY DID GOD COMMAND THE CIRCUMCISION OF THE CHILDREN ON THE EIGHTH DAY? — God commanded to circumcise every male child on the eighth day after his birth, because after the number of seven days had passed, the eighth day became like the first after the Sabbath, and was no longer the eighth, but the first. As the salvation which Christ was to bring to the world was to take place on the first day called the day of the Lord, because the Lord resurrected that day after the Sabbath; the figurative sign of salvation was given in circumcision to make known the future regeneration even under the law of circumcision. It was by faith that men were to be saved, and this faith received its confirmation on the first day of the resurrection of the Savior; the figurative sign of salvation was thus established on the first day in the law of circumcision, which was also to be the sign of the faith of Abraham. Harmony is therefore complete here; the sign of the past faith which was given on the first day, was the figure of the future faith which was firmly established on the first day. [Questions on the Old and New Testaments, PL 35,

In what sense is the covenant eternal? One interpretation is that it is eternal according to the One promising, for the things of God are not time conditioned. But relative to us eternal things become time conditioned. Another interpretation is that even when the covenant with Israel was abolished, it was maintained for us, and we are God's [people] in place of them. Circumcision took over a second territory after the faith [came]. For just as the birth of the illegitimate child came first, after the promise of the legitimate one, because the birth of the latter was likewise belated, so now circumcision came first because the time for the spiritual [circumcision] was not yet; but when this new one appears, the other circumcision is thrown out. For "in Christ Jesus," we say, "neither circumcision nor uncircumcision is of any avail." That circumcision was a sign of the covenant, which could be present even in transgressors. But the true circumcision is the perfect observance of the law, the cutting away and removing of everything alien to God and the ability to pass beyond worldly things to approach the transcendent realities through understanding. Of this the eighth day of circumcision is the symbol. For the eighth day is supernatural, even as the Savior, by accomplishing the resurrection on the eighth day, showed the mystery. Likewise appropriate is the text's reference to extermination, whether of those uncircumcised in the flesh or those whose heart has not been circumcised, the uncircumcised of heart, as one would say using Old Testament terminology.