1 Faustus said: Assuredly I believe the apostle. And yet I do not believe that the Son of God was born of the seed of David according to the flesh, because I do not believe that God's apostle could contradict himself, and have one opinion about our Lord at one time, and another at another. But, granting that he wrote this—since you will not hear of anything being spurious in his writings—it is not against us. For this seems to be Paul's old belief about Jesus, when he thought, like everybody else, that Jesus was the son of David.
Afterwards, when he learned that this was false, he corrects himself; and in his Epistle to the Corinthians he says: "We know no man after the flesh; yea, though we have known Christ after the flesh, yet now henceforth know we Him no more." Observe the difference between these two verses. In one he asserts that Jesus was the son of David after the flesh; in the other he says that now he knows no man after the flesh. If Paul wrote both, it can only have been in the way I have stated.
In the next verse he adds: "Therefore, if any man be in Christ, he is a new creature; old things are passed away; behold, all things have become new." The belief that Jesus was born of the seed of David according to the flesh is of this old transitory kind; whereas the faith which knows no man after the flesh is new and permanent. So, he says elsewhere: "When I was a child, I spoke as a child, I understood as a child, I thought as a child; but when I became a man, I put away childish things." We are thus warranted in preferring the new and amended confession of Paul to his old and faulty one.
And if you hold by what is said in the Epistle to the Romans, why should not we hold by what is said to the Corinthians? But it is only by your insisting on the correctness of the text that we are made to represent Paul as building again the things which he destroyed, in spite of his own repudiation of such prevarication. If the verse is Paul's, he has corrected himself. If Paul should not be supposed to have written anything requiring correction, the verse is not his.
Source: Reply to Faustus the Manichaean (New Advent)