7 Hence, as you receive nothing in the Old Testament except the prophecies and the common precepts of practical morality, which we quoted above, while you set aside circumcision, and sacrifices, and the Sabbath and its observance, and the feast of unleavened bread, why should not we receive nothing in the New Testament but what we find said in honor and praise of the majesty of the Son, either by Himself or by His apostles, with the proviso, in the case of the apostles, that it was said by them after reaching perfection, and when no longer in unbelief; while we take no notice of the rest, which, if said at the time, was the utterance of ignorance or inexperience, or, if not, was added by crafty opponents with a malicious intention, or was stated by the writers without due consideration, and so handed down as authentic?
Take as examples, the shameful birth of Jesus from a woman, His being circumcised like the Jews, His offering sacrifice like the Gentiles, His being baptized in a humiliating manner, His being led about by the devil in the wilderness, and His being tempted by him in the most distressing way. With these exceptions, besides whatever has been inserted under the pretence of being a quotation from the Old Testament, we believe the whole, especially the mystic nailing to the cross, emblematic of the wounds of the soul in its passion; as also the sound moral precepts of Jesus, and His parables, and the whole of His immortal discourse, which sets forth especially the distinction of the two natures, and therefore must undoubtedly be His.
There is, then, no reason for your thinking it obligatory in me to believe all the contents of the Gospels; for you, as has been proved, take so dainty a sip from the Old Testament, that you hardly, so to speak, wet your lips with it.
Source: Reply to Faustus the Manichaean (New Advent)