Celsus, after what has been said, goes on as follows: “I can tell how the very thing occurred, viz., that they should call him 'Son of God.' Men of ancient times termed this world, as being born of God, both his child and his son. Both the one and other 'Son of God,' then, greatly resembled each other.” He is therefore of opinion that we employed the expression “Son of God,” having perverted what is said of the world, as being born of God, and being His “Son,” and “a God.” For he was unable so to consider the times of Moses and the prophets, as to see that the Jewish prophets predicted generally that there was a “Son of God” long before the Greeks and those men of ancient time of whom Celsus speaks.
Nay, he would not even quote the passage in the letters of Plato, to which we referred in the preceding pages, concerning Him who so beautifully arranged this world, as being the Son of God; lest he too should be compelled by Plato, whom he often mentions with respect, to admit that the architect of this world is the Son of God, and that His Father is the first God and Sovereign Ruler over all things. Nor is it at all wonderful if we maintain that the soul of Jesus is made one with so great a Son of God through the highest union with Him, being no longer in a state of separation from Him.
For the sacred language of holy Scripture knows of other things also, which, although “dual” in their own nature, are considered to be, and really are, “one” in respect to one another. It is said of husband and wife, “They are no longer two, but one flesh;” and of the perfect man, and of him who is joined to the true Lord, Word, and Wisdom, and Truth, that “he who is joined to the Lord is one spirit.” And if he who “is joined to the Lord is one spirit,” who has been joined to the Lord, the Very Word, and Wisdom, and Truth, and Righteousness, in a more intimate union, or even in a manner at all approaching to it than the soul of Jesus? And if this be so, then the soul of Jesus and God the Word— the first-born of every creature— are no longer two, (but one).
Source: Against Celsus (New Advent)