Faustus warns against pressing too far the argument, that if Jesus was not born He cannot have suffered. Augustine accepts the birth and death alike on the testimony of the Gospel narrative, which is higher authority than the falsehood of Manichæus.
1. Faustus said: If Jesus was not born, He cannot have suffered; but since He did suffer, He must have been born. I advise you not to have recourse to logical inference in these matters, or else your whole faith will be shaken. For, even according to you, Jesus was born miraculously of a virgin; which the argument from consequents to antecedents shows to be false. For your argument might thus be turned against you: If Jesus was born of a woman, He must have been begotten by a man; but He was not begotten by a man, therefore He was not born of a woman. If, as you believe, He could be born without being begotten, why could He not also suffer without being brought forth?
2. Augustine replied: The argument which you here reply to is one which could be used only by such ignorant people as you succeed in misleading, not by those who know enough to refute you. Jesus could both be born without being begotten and suffer without being brought forth. His being one and not the other was the effect of His own will. He chose to be born without being begotten, and not to suffer without being brought forth. And if you ask how I know that He was brought forth, and that He suffered, I read this in the faithful Gospel narrative. If I ask how you know what you state, you bring forward the authority of Manichæus, and charge the Gospel with falsehood. Even if Manichæus did not set forth falsehood as an excellence in Christ, I should not believe his statements. His praise of falsehood comes from nothing that he found in Christ, but from his own moral character.
Source: Reply to Faustus the Manichaean (New Advent)