The Eastern Orthodox concept of theosis is much maligned and mis-understood. I was startled to find no less an authority than Charles Spurgeon waxing eloquent on the topic, sounding like an Orthodox church father:
“Partakers of the divine nature.”—2 Peter 1:4.
“To be a partaker of the divine nature is not, of course, to become God. That cannot be. The essence of Deity is not to be participated in by the creature. Between the creature and the Creator there must ever be a gulf fixed in respect of essence; but as the first man Adam was made in the image of God, so we, by the renewal of the Holy Spirit, are in a yet diviner sense made in the image of the Most High, and are partakers of the divine nature. We are, by grace, made like God. “God is love”; we become love—”He that loveth is born of God.” God is truth; we become true, and we love that which is true: God is good, and He makes us good by His grace, so that we become the pure in heart who shall see God. Moreover, we become partakers of the divine nature in even a higher sense than this—in fact, in as lofty a sense as can be conceived, short of our being absolutely divine. Do we not become members of the body of the divine person of Christ? Yes, the same blood which flows in the head flows in the hand: and the same life which quickens Christ quickens His people, for “Ye are dead, and your life is hid with Christ in God.” Nay, as if this were not enough, we are married unto Christ. He hath betrothed us unto Himself in righteousness and in faithfulness, and he who is joined unto the Lord is one spirit. Oh! marvellous mystery! we look into it, but who shall understand it? One with Jesus—so one with Him that the branch is not more one with the vine than we are a part of the Lord, our Saviour, and our Redeemer! While we rejoice in this, let us remember that those who are made partakers of the divine nature will manifest their high and holy relationship in their intercourse with others, and make it evident by their daily walk and conversation that they have escaped the corruption that is in the world through lust. O for more divine holiness of life!” (Charles Spurgeon, Morning By Morning)
A most apt way to describe what Orthodox Christians mean by theosis!
To learn more about what Orthodox Christians really believe about theosis, I would recommend Light from the Christian East (ch. 8), Eastern Orthodoxy Through Western Eyes (ch. 6), and Eastern Orthodoxy Christianity: A Western Perspective (ch. 6).