2 If there are parts of the Testament of the Father which we are not bound to observe (for you attribute the Jewish law to the Father, and it is well known that many things in it shock you, and make you ashamed, so that in heart you no longer regard it as free from corruption, though, as you believe, the Father Himself partly wrote it for you with His own finger while part was written by Moses, who was faithful and trustworthy), the Testament of the Son must be equally liable to corruption, and may equally well contain objectionable things; especially as it is allowed not to have been written by the Son Himself, nor by His apostles, but long after, by some unknown men, who, lest they should be suspected of writing of things they knew nothing of, gave to their books the names of the apostles, or of those who were thought to have followed the apostles, declaring the contents to be according to these originals.
In this, I think, they do grievous wrong to the disciples of Christ, by quoting their authority for the discordant and contradictory statements in these writings, saying that it was according to them that they wrote the Gospels, which are so full of errors and discrepancies, both in facts and in opinions, that they can be harmonized neither with themselves nor with one another. This is nothing else than to slander good men, and to bring the charge of dissension on the brotherhood of the disciples.
In reading the Gospels, the clear intention of our heart perceives the errors, and, to avoid all injustice, we accept whatever is useful, in the way of building up our faith, and promoting the glory of the Lord Christ, and of the Almighty God, His Father, while we reject the rest as unbecoming the majesty of God and Christ, and inconsistent with our belief.
Source: Reply to Faustus the Manichaean (New Advent)