"We become united with God by grace in the Person of Christ, who is God come in the flesh. The means of becoming “like God” is through perfection in holiness, the continuous process of acquiring the Holy Spirit by grace through ascetic devotion. Some Protestants might refer to this process as sanctification. Another term for it, perhaps more familiar to Western Christians, would be mortification—putting sin to death within ourselves."
"With the Incarnation, God has assumed and glorified our flesh and has consecrated and sanctified our humanity. He has also given us the Holy Spirit. As we acquire more of the Holy Spirit in our daily lives, we become more like Christ, and we have the opportunity of being granted, in this life, illumination or glorification. When we speak of acquiring more of the Holy Spirit, it is in the sense of appropriating to a greater degree what has actually been given to us already by God. We acquire more of what we are more able to receive. God the Holy Spirit remains ever constant."
It is also the focal point of Jesus' prayer in John 17:
20 "I ask not only on behalf of these, but also on behalf of those who will believe in me through their word, 21 that they may all be one. As you, Father, are in me and I am in you, may they also be in us, so that the world may believe that you have sent me. 22 The glory that you have given me I have given them, so that they may be one, as we are one, 23 I in them and you in me, that they may become completely one, so that the world may know that you have sent me and have loved them even as you have loved me. 24 Father, I desire that those also, whom you have given me, may be with me where I am, to see my glory, which you have given me because you loved me before the foundation of the world.
This is a mystery, an extension of the mystery of the Trinity. That God might be One, while at the same time three persons, is now also reflected in our being One with God, united with him through the person of Christ, while at the same time remaining distinct from the Father and the very being of God.
The easiest framework within which we might conceptualize this mystery is that of a Divine Family, and our status of 'Children of God'.
We contemplate this Christmas, as we do every Christmas, the miracle of God incarnate, cradled in a manger. The Word become flesh, dwelling among us.
Equally miraculous will be that day when our flesh will be redeemed, and by the power of the Spirit we will be cradled in union with God.
Christ was with us where we are, in order that we might be with him where he is".