3 Assuredly, if the Manichæans were justified by the resurrection of the Lord,— the day of whose resurrection, the third after His passion, was the eighth day, coming after the Sabbath, that is, after the seventh day—their carnal minds would be delivered from the darkness of earthly passions which rests on them; and rejoicing in the circumcision of the heart, they would not ridicule it as prefigured in the Old Testament by circumcision in the flesh, although they should not enforce this observance under the New Testament.
But, as the apostle says, "To the pure all things are pure. But to the impure and unbelieving nothing is pure, but both their mind and conscience are defiled." So these people, who are so pure in their own eyes, that they regard, or pretend to regard, as impure these members of their bodies, are so defiled with unbelief and error, that, while they abhor the circumcision of the flesh—which the apostle calls a seal of the righteousness of faith—they believe that the divine members of their God are subjected to restraint and contamination in these very carnal members of theirs.
For they say that flesh is unclean; and it follows that God, in the part which is detained by the flesh, is made unclean: for they declare that He must be cleansed, and that till this is done, as far as it can be done, He undergoes all the passions to which flesh is subject, not only in suffering pain and distress, but also in sensual gratification. For it is for His sake, they say, that they abstain from sexual intercourse, that He may not be bound more closely in the bondage of the flesh, nor suffer more defilement.
The apostle says, "To the pure all things are pure." And if this is true of men, who may be led into evil by a perverse will, how much more must all things be pure to God, who remains for ever immutable and immaculate! In those books which you defile with your violent reproaches, it is said of the divine wisdom, that "no defiled thing falls into it, and it goes everywhere by reason of its pureness." It is mere prurient absurdity to find fault with the sign of human regeneration appointed by that God, to whom all things are pure, to be put on the organ of human generation, while you hold that your God, to whom nothing is pure, is in a part of his nature subjected to taint and corruption by the vicious actions in which impure men employ the members of their body.
For if you think there is pollution in conjugal intercourse, what must there be in all the practices of the licentious? If you ask, then, as you often do, whether God could not find some other way of sealing the righteousness of faith, the answer is, Why not this way, since all things are pure to the pure, much more to God? And we have the authority of the apostle for saying that circumcision was the seal of the righteousness of the faith of Abraham. As for you, you must try not to blush when you are asked whether your God had nothing better to do than to entangle part of his nature with these members that you revile so much.
These are delicate subjects to speak of, on account of the penal corruption attending the propagation of man. They are things which call into exercise the modesty of the chaste, the passions of the impure, and the justice of God.
Source: Reply to Faustus the Manichaean (New Advent)