Celsus, however, asserts that the answer which we give is based upon a probable conjecture, admitting that he describes our answer in the following terms: “Since God is great and difficult to see, He put His own Spirit into a body that resembled ours, and sent it down to us, that we might be enabled to hear Him and become acquainted with Him.” But the God and Father of all things is not the only being that is great in our judgment; for He has imparted (a share) of Himself and His greatness to His Only-begotten and First-born of every creature, in order that He, being the image of the invisible God, might preserve, even in His greatness, the image of the Father.
For it was not possible that there could exist a well-proportioned, so to speak, and beautiful image of the invisible God, which did not at the same time preserve the image of His greatness. God, moreover, is in our judgment invisible, because He is not a body, while He can be seen by those who see with the heart, that is, the understanding; not indeed with any kind of heart, but with one which is pure. For it is inconsistent with the fitness of things that a polluted heart should look upon God; for that must be itself pure which would worthily behold that which is pure.
Let it be granted, indeed, that God is “difficult to see,” yet He is not the only being who is so; for His Only-begotten also is “difficult to see.” For God the Word is “difficult to see,” and so also is His wisdom, by which God created all things. For who is capable of seeing the wisdom which is displayed in each individual part of the whole system of things, and by which God created every individual thing? It was not, then, because God was “difficult to see” that He sent God His Son to be an object “easy to be seen.” And because Celsus does not understand this, he has represented us as saying, “Because God was 'difficult to see,' He put His own Spirit in a body resembling ours, and sent it down to us, that we might be enabled to hear Him and become acquainted with Him.”
Now, as we have stated, the Son also is “difficult to see,” because He is God the Word, through whom all things were made, and who “tabernacled among us.”
Source: Against Celsus (New Advent)