The illustrious Celsus, taking occasion I know not from what, next raises an additional objection against us, as if we asserted that “God Himself will come down to men.” He imagines also that it follows from this, that “He has left His own abode;” for he does not know the power of God, and that “the Spirit of the Lord fills the world, and that which upholds all things has knowledge of the voice.” Nor is he able to understand the words, “Do I not fill heaven and earth? Says the Lord.” Nor does he see that, according to the doctrine of Christianity, we all “in Him live, and move, and have our being,” as Paul also taught in his address to the Athenians; and therefore, although the God of the universe should through His own power descend with Jesus into the life of men, and although the Word which was in the beginning with God, which is also God Himself, should come to us, He does not give His place or vacate His own seat, so that one place should be empty of Him, and another which did not formerly contain Him be filled.
But the power and divinity of God comes through him whom God chooses, and resides in him in whom it finds a place, not changing its situation, nor leaving its own place empty and filling another: for, in speaking of His quitting one place and occupying another, we do not mean such expressions to be taken topically; but we say that the soul of the bad man, and of him who is overwhelmed in wickedness, is abandoned by God, while we mean that the soul of him who wishes to live virtuously, or of him who is making progress (in a virtuous life), or who is already living conformably thereto, is filled with or becomes a partaker of the Divine Spirit.
It is not necessary, then, for the descent of Christ, or for the coming of God to men, that He should abandon a greater seat, and that things on earth should be changed, as Celsus imagines when he says, “If you were to change a single one, even the least, of things on earth, all things would be overturned and disappear.” And if we must speak of a change in any one by the appearing of the power of God, and by the entrance of the word among men, we shall not be reluctant to speak of changing from a wicked to a virtuous, from a dissolute to a temperate, and from a superstitious to a religious life, the person who has allowed the word of God to find entrance into his soul.
Source: Against Celsus (New Advent)