... Orthodoxy is expected to do something. It's not just a matter of having the right theology, or the right ecclesiastical institution (though I believe it does have those things). It's that there is an innate dynamism. This faith is expected to accomplish something—to enable a person to grow in union with God....
The Eastern Christian Way includes the elements we'd associate with any spiritual path—prayer, fasting, almsgiving, and so forth—and they fit together organically, like the parts of a diet-and-exercise program. Somebody who doesn't actually do the work and put it into practice won't see any benefit (another way it's like a diet-and-exercise program). But for those who do, transformation takes place. It happens in ways that are notable and recognizable, and even, in some cases, miraculous....
Wherever the faith is taken, this Way takes root and bears fruit....
The goal of this healing path is union with God. This is called theosis, which is usually translated "deification" or "divinization...." We can dismantle the Greek word and see that it is composed of theos, which means "God," and the suffix –osis, which indicates a process. As red dye saturates a white cloth by the process of osmosis, so humans can be saturated with God's presence by the process of theosis....
We can see this transformation in the Gospel story of Christ's Transfiguration. He took Peter, James, and John aside and led them up a high mountain. "And he was transfigured before them, and his face shone like the sun, and his garments became as white as light.... A bright cloud overshadowed them, and a voice from the cloud said, 'This is my beloved Son, with whom I am well pleased, listen to him.' When the disciples heard this, they fell on their faces, and were filled with awe. But Jesus came and touched them, saying, 'Rise and have no fear'" (Matt. 17:1–8).
This was not a change in Jesus; he had always been filled with glory. It was a change in the disciples' ability to perceive.... The Orthodox hymn for the Transfiguration says that Christ revealed his glory "as far as they [the disciples] could bear it...."
The light of the Transfiguration is not ordinary earthly light, but the light of God's glory, the light that was before the universe was made, called the "Uncreated Light" in Orthodoxy. God made us like himself ("image and likeness") so that we could take on this light as a lump of coal takes on fire. This is the destiny we were created for: participation in the light and glory of God. This expectation is the catalyst of Orthodox spirituality….
The miracle of human clay bearing divine fire is foreshadowed in the burning bush (Exod. 3:1–6). God's presence wholly irradiates a dry desert shrub, but does not destroy it. Miraculously, the bush remains intact, remains itself. God's presence doesn't obliterate or replace us, but helps us to become ourselves—each of us the real self he has always intended us to be....
Theosis is not something we achieve by trying really hard. We require a more radical kind of surgery; in fact, we must die. As we die to self and get out of the way, Christ's life can fill us. "You have died, and your life is hid with Christ in God" (Col. 3:3).
Frederica Mathewes-Green, Welcome to the Orthodox Church: An Introduction to Eastern Christianity (Brewster, Massachusetts: Paraclete Press, 2015), 67–71.