22 As regards not swearing, in which also you say that Christ completed the law given to these righteous men of antiquity, I cannot be certain that they did not swear, for we find that Paul the apostle swore. With you, swearing is still a common practice, for you swear by the light, which you love as flies do; for the light of the mind which lights every man that comes into the world, as distinct from mere natural light, you know nothing of. You swear, too, by your master Manichæus, whose name in his own tongue was Manes.
As the name Manes seemed to be connected with the Greek word for madness, you have changed it by adding a suffix, which only makes matters worse, by giving the new meaning of pouring forth madness. One of your own sect told me that the name Manichæus was intended to be derived from the Greek words for pouring forth manna; for χέειν means to pour. But, as it is, you only express the idea of madness with greater emphasis. For by adding the two syllables, while you have forgotten to insert another letter in the beginning of the word, you make it not Mannichæus, but Manichæus; which must mean that he pours forth madness in his long unprofitable discourses.
Again, you often swear by the Paraclete,— not the Paraclete promised and sent by Christ to His disciples, but this same madness-pourer himself. Since, then, you are constantly swearing, I should like to know in what sense you make Christ to have fulfilled this part of the law, which is one you mention as belonging to the earliest times. And what do you make of the oaths of the apostle? For as to your authority, it cannot weigh much with yourselves, not to speak of me or any other person.
It is therefore evident that Christ's words, "I have come not to destroy the law, but to fulfill it," have not the meaning which you give them. Christ makes no reference in these words to His comments on the ancient sayings which He quotes, and of which His discourse was an explanation, but not a fulfillment.
Source: Reply to Faustus the Manichaean (New Advent)