3 It is amazingly bold in the impious and impure sect of the Manichæans to boast of being the chaste bride of Christ. All the effect of such a boast on the really chaste members of the holy Church is to remind them of the apostle's warning against deceivers: "I have joined you to one husband, to present you as a chaste virgin to Christ. But I fear lest, as the serpent deceived Eve by his guile, so your minds also should be corrupted from the purity which is in Christ." What else do those preachers of another gospel than that which we have received try to do, but to corrupt us from the purity which we preserve for Christ, when they stigmatize the law of God as old, and praise their own falsehoods as new, as if all that is new must be good, and all that is old bad?
The Apostle John, however, praises the old commandment, and the Apostle Paul bids us avoid novelties in doctrine. As an unworthy son and servant of the Catholic Church, the true bride of the true Christ, I too, as appointed to give out food to my fellow-servants, would speak to her a word of counsel. Continue ever to shun the profane errors of the Manichæans, which have been tried by the experience of your own children, and condemned by their recovery. By that heresy I was once separated from your fellowship, and after running into danger which ought to have been avoided, I escaped.
Restored to your service, my experience may perhaps be profitable to you. Unless your true and truthful Bridegroom, from whose side you were made, had obtained the remission of sins through His own real blood, the gulf of error would have swallowed me up; I should have become dust, and been devoured by the serpent. Be not misled by the name of truth. The truth is in your own milk, and in your own bread. They have the name only, and not the thing. Your full-grown children, indeed, are secure; but I speak to your babes, my brothers, and sons, and masters, whom you, the virgin mother, fertile as pure, cherishes into life under your anxious wings, or nourishes with the milk of infancy.
I call upon these, your tender offspring, not to be seduced by noisy vanities, but rather to pronounce accursed any one that preaches to them another gospel than that which they have received in you. I call upon these not to leave the true and truthful Christ, in whom are hid all the treasures of wisdom and knowledge; not to forsake the abundance of His goodness which He has laid up for them that fear Him, and has wrought for them that trust in Him. How can they expect to find truthful words in one who preaches an untruthful Christ? Scorn the reproaches cast on you, for you know well that the gift which you desire from your Bridegroom is eternal life, for He Himself is eternal life.
Source: Reply to Faustus the Manichaean (New Advent)