The Ascension is a very important feast for Orthodox spirituality.
In Orthodox theology, the person of the Word of God became flesh, taking to that person of the Word of God a full human nature. The person was and is the Second Person of the Holy Trinity from all eternity with the Father and the Holy Spirit in unspeakable communion. This unspeakable communion Elder Porphyrios calls in Wounded by Love the Uncreated Church. That Second Person of the Holy Trinity took on our complete human nature—yet without sin—when he was conceived as a man in the womb of Mary. As was announced by the Archangel Gabriel, he was given at birth and circumcision the name of Jesus. Jesus, who is called Christ, is the Word of God made flesh. He has two natures in one person, his divine and his human nature. The person remains the Word of God, now made flesh—now having taken on a complete human nature.
We are emphasizing this Christology so as to make clear the importance of the Ascension. Because the person of Jesus Christ is always the Second Person of the Holy Trinity, his divine nature did not need to ascend to the Father. It was and is always with the Father. “I and my Father are one.” However, this is not the case with the human nature of Christ.
We learn from St Basil the Great that the human nature of Christ was divinized at the moment of Jesus' conception in the womb of Mary (March 25). This means that from the instant of his conception in the womb of Mary, Jesus was a perfect man, the second Adam, a man like us in all things but sin. But this is the key to our understanding the Ascension. Because Jesus was a perfect man, an undefiled image and likeness of the Father, the Ascension must mean something more. And that something more is the difference between our vocation as Christians, and the state of Adam and Eve in Paradise. We are called to ascend with Jesus through the grace of the Holy Spirit sent by the Father (as much as is humanly possible). Hence, our divinization does not stop with the restoration of the likeness to God that Adam and Eve had in Paradise, but continues in our own ascension, especially after our earthly life, in death and the General Resurrection.
What is is the nature of divinization? This is a difficult concept for the non-Orthodox to grasp. Essentially divinization has to do with the image or icon of God that man was created and intended to be. To be divinized means to attain to the properties (characteristics, moral and spiritual virtues) of God—the likeness to God that Adam and Eve lost by their disobedience—through the grace of God, through the uncreated grace of God that the Orthodox identify with the Holy Spirit and call the 'uncreated energies' of God. To be divinized means to enter into intimacy with God. But this intimacy of man with God is not a distant similarity only, but includes the concept of a union that is portrayed in Holy Scripture by the image of marriage. Hence, to be divinized is to marry God. It is to be like unto God. It is to become a friend of God. But as the Fathers point out, it is to become all of these things without becoming God by nature.
When in the Ascension the human nature of Jesus Christ ascended and sat at the right hand of the Father, the human nature of Jesus reached the fullness of this intimacy or union with God in a relationship of love and gnosis. And this ascension of Jesus Christ opened the way for all men, all those who share in the same human nature, all those who have been baptized, to ascend to the same intimacy with God (as much as is humanly possible).