6 With regard, then, to this apparent contradiction between the passage which speaks of the Son of God being of the seed of David, to the words, "Though we have known Christ after the flesh, yet now henceforth know we Him no more," even though both quotations were not from the writings of one apostle,— though one were from Paul, and the other from Peter, or Isaiah, or any other apostle or prophet—such is the equality of canonical authority, that it would not be allowable to doubt of either.
For the utterances of Scripture, harmonious as if from the mouth of one man, commend themselves to the belief of the most accurate and clear-sighted piety, and demand for their discovery and confirmation the calmest intelligence and the most ingenious research. In the case before us both quotations are from the canonical, that is, the genuine epistles of Paul. We cannot say that the manuscript is faulty, for the best Latin translations substantially agree; or that the translations are wrong, for the best texts have the same reading.
So that, if any one is perplexed by the apparent contradiction, the only conclusion is that he does not understand. Accordingly it remains for me to explain how both passages, instead of being contradictory, may be harmonized by one rule of sound faith. The pious inquirer will find all perplexity removed by a careful examination.
Source: Reply to Faustus the Manichaean (New Advent)