5 There is no need, then that you should extol so much the perfection of Christ's commands, because you obey the precepts of the gospel. For the precepts, supposing you really to fulfill them, would not profit you without true faith. Do you not know that the apostle says, "If I distribute all my goods to the poor, and give my body to be burned, and have not charity, it profits me nothing?" Why do you boast of having Christian poverty, when you are destitute of Christian charity?
Robbers have a kind of charity to one another, arising from a mutual consciousness of guilt and crime; but this is not the charity commended by the apostle. In another passage he distinguishes true charity from all base and vicious affections, by saying, "Now the end of the commandment is charity out of a pure heart, and a good conscience, and faith unfeigned." How then can you have true charity from a fictitious faith? You persist in a faith corrupted by falsehood: for your First Man, according to you, used deceit in the conflict by changing his form, while his enemies remained in their own nature; and, besides, you maintain that Christ, who says, "I am the truth," feigned His incarnation, His death on the cross, the wounds of His passion, the marks shown after His resurrection.
If you speak the truth, and your Christ speaks falsehood, you must be better than he. But if you really follow your own Christ, your truthfulness may be doubted, and your obedience to the precepts you speak of may be only a pretence. Is it true, as Faustus says, that you have no money in your purses? He means, probably, that your money is in boxes and bags; nor would we blame you for this, if you did not profess one thing and practise another. Constantius, who is still alive, and is now our brother in Catholic Christianity, once gathered many of your sect into his house at Rome, to keep these precepts of Manichæus, which you think so much of, though they are very silly and childish.
The precepts proved too much for your weakness, and the gathering was entirely broken up. Those who persevered separated from your communion, and are called Mattarians, because they sleep on mats,— a very different bed from the feathers of Faustus and his goatskin coverlets, and all the grandeur that made him despise not only the Mattarians, but also the house of his poor father in Mileum. Away, then, with this accursed hypocrisy from your writing, if not from your conduct; or else your language will conflict with your life by your deceitful words, as your First Man with the race of darkness by his deceitful elements.
Source: Reply to Faustus the Manichaean (New Advent)