As a student of Pope St Leo the Great and the Council of Chalcedon — and, thus, its aftermath — the significance of that Councils’ Symbolon of the Christian Faith, its definition (which I translated here), is often just below the surface of my mind. Thus, I greatly appreciate this from Metropolitan Anthony Bloom, God and Man:
There is in the Church a vision of man which is not a theory of man. It is not the ideal man, it is not the invented man, nor man as we wish he were, and towards which we aim as a sort of created transcendence. No, we have put on the altar a concrete real man — Jesus of Nazareth — and we must have a look at what is implied. We see in the Creed that Christ was true man and true God. When we say that He was true man we imply two thing: the fact that He was God has not made Him into a man alien to us, a man so different from us that He has only the same shape and the same name while in reality He has nothing in common with us; on the other hand, we proclaim that being the true man means to be a revelation of man in his fulfilment, man as he is called to be, and that in Christ we have a vision — concrete, real, historical — of what we are called to become in our realilty, in our historicity and in our becoming. So when we say that Christ is true man, we affirm that to be united with the Godhead does not annihilate or change the nature of man, and it is only in Him, because man is united in Him with the Godhead, that man is revealed in his full potentiality. Because man as a specimen of natural history i snot man in the sense in which we believe man is truly human. Man becomes truly human only when he is united with God intimately, deeply, inseparably, so that the fullness of Godhead abides in the flesh. I am using terms which are applied to Christ in the Scriptures, but which I believe are applicable to man if we take, for instance, the words of St Peter in his Epistle that our vocation is to become partakers of the divine nature — God’s participators and not just human beings related to a God who remains an outsider to us. But that implies a quite different vision of man, and it also implies something which I believe to be important, a quite different vision of the Church. (pp. 60-61)
True humanity is only fully realised in union with God. This is, at one level, the Adamic state (did not God walk in the Garden in the cool of the evening?). At another level, it is something higher. Many of the Church Fathers believe that the human race was meant to progress in knowledge of God and perfection even without the Fall, but that sin now hampers us (Sts Irenaeus and Athanasius, at least). Christ, by uniting humanity to the Divine, has reignited our ability to be who we are meant to be — and to go beyond even Adam.
Here we also have a good description of the Orthodox doctrine of theosis. Met. Anthony here references 2 Peter 1:3-4:
According as his divine power hath given unto us all things that pertain unto life and godliness, through the knowledge of him that hath called us to glory and virtue: 4 Whereby are given unto us exceeding great and precious promises: that by these ye might be partakers of the divine nature, having escaped the corruption that is in the world through lust.
This reminds me of St Leo, in fact; Leo argues that because of the Incarnation and the full humanity of Christ who is also fully God, we enter into the divine relationship — and we have a duty to our human neighbours who are sharers in the same nature as Christ. And Christ is God.
A friend recently expressed doubt about the possibility of theosis reconciling itself with Scripture. Theosis is about union with God where we retain all of our humanity but share in the divine nature by God’s grace. It is based on passages like 2 Peter 1, or Ephesians 4:13, or Romans 8:29, or 2 Corinthians 3:18. It is also, when properly understood (I recommend Met. Kallistos on theosis), an implication or outgrowing of the Church’s dogmatic statements in the Seven Ecumenical Councils, the Rule of Faith, and the historic liturgies.
Theosis rests, in western terminology, on both Scripture and Tradition. (So we Anglicans can accept it when it is properly understood.)
This is, of course, the goal of mysticism and asceticism:
Release me, and free my heart from all dependence on the passing consolation of wicked things, since none of these things can yield true satisfaction or appease my longings. Unite me to Yourself by the unbreakable bonds of love. You alone can satisfy the soul that loves You, and without You the world is worthless. -St Thomas à Kempis, The Imitation of Christ, 3.23, trans. Leo Sherley-Price, p. 124
Let us, therefore, seek the Face of Christ, enter into God’s throneroom, and, resting in the stillness, become partakers of the Divine Life. This is the greatest implication of Chalcedon for the Christian life today. Own it. Live it.