In order to avoid misunderstanding what late antique Christian theologians mean (and do not mean) when they discuss “theosis” or “deification” (and some of their statements read without context are very open to misunderstanding), it is of the utmost importance to recall that orthodox fourth-century theologians rigorously and emphatically affirmed the absolute and essential (I use this word with its full etymological force) distinction between God as Creator and all created nature.
One instance of such an affirmation that is very clear indeed is found in Gregory of Nazianzus’ second Theological Oration:
What is this that has happened to me, O friends, and initiates, and fellow-lovers of the truth? I was running to lay hold on God, and thus I went up into the Mount, and drew aside the curtain of the Cloud, and entered away from matterand material things, and as far as I could I withdrew within myself. And then when I looked up, I scarce saw the back parts of God; Exodus 33:23 although I was sheltered by the Rock, the Word that was made flesh for us. And when I looked a little closer, I saw, not the First and unmingled Nature, known to Itself— to theTrinity, I mean; not That which abides within the first veil, and is hidden by theCherubim; but only that Nature, which at last even reaches to us. And that is, as far as I can learn, the Majesty, or as holy David calls it, the Glory which is manifested among the creatures, which It has produced and governs. For these are the Back Parts of God, which He leaves behind Him, as tokens of Himself like the shadows and reflection of the sun in the water, which show the sun to our weak eyes, because we cannot look at the sun himself, for by his unmixed light he is too strong for our power of perception. In this way then shall you discourse of God; even were thou a Moses and a god to Pharaoh; Exodus 4:2 even were thou caught up like Paul to the Third Heaven, 2 Corinthians 12:2 and had heard unspeakable words; even were thou raised above them both, and exalted to Angelic or Archangelic place and dignity. For though a thing be all heavenly, or above heaven, and far higher in nature and nearer to God than we, yet it is farther distant from God, and from the complete comprehension of His Nature, than it is lifted above our complex and lowly and earthward sinking composition. (Oration 28.3)