“But that certain Christians and (all) Jews should maintain, the former that there has already descended, the latter that there will descend, upon the earth a certain God, or Son of a God, who will make the inhabitants of the earth righteous, is a most shameless assertion, and one the refutation of which does not need many words.” Now here he appears to pronounce correctly regarding not “certain” of the Jews, but all of them, that they imagine that there is a certain (God) who will descend upon the earth; and with regard to Christians, that certain of them say that He has already come down.
For he means those who prove from the Jewish Scriptures that the advent of Christ has already taken place, and he seems to know that there are certain heretical sects which deny that Christ Jesus was predicted by the prophets. In the preceding pages, however, we have already discussed, to the best of our ability, the question of Christ having been the subject of prophecy, and therefore, to avoid tautology, we do not repeat much that might be advanced upon this head. Observe, now, that if he had wished with a kind of apparent force to subvert faith in the prophetic writings, either with regard to the future or past advent of Christ, he ought to have set forth the prophecies themselves which we Christians and Jews quote in our discussions with each other.
For in this way he would have appeared to turn aside those who are carried away by the plausible character of the prophetic statements, as he regards it, from assenting to their truth, and from believing, on account of these prophecies, that Jesus is the Christ; whereas now, being unable to answer the prophecies relating to Christ, or else not knowing at all what are the prophecies relating to Him, he brings forward no prophetic declaration, although there are countless numbers which refer to Christ; but he thinks that he prefers an accusation against the prophetic Scriptures, while he does not even state what he himself would call their “plausible character!”
He is not, however, aware that it is not at all the Jews who say that Christ will descend as a God, or the Son of a God, as we have shown in the foregoing pages. And when he asserts that “he is said by us to have already come, but by the Jews that his advent as Messiah is still future,” he appears by the very charge to censure our statement as one that is most shameless, and which needs no lengthened refutation.
Source: Against Celsus (New Advent)