Norman Russell, in his book Fellow Workers with God:  Orthodox thinking on theosis (St Vladimir's Seminary Press, 2009) discusses how the Transfiguration of Jesus is a theophany, or manifestation of God.  The excerpt appears on pp 102-104.

+++++++++++++++++++++++++++++++++++++++++++++++++++++++

"What does it mean, 'he was transfigured'?  It means he allowed a brief glimpse of the Godhead and showed them the indwelling God."  This text from St John Chrysostom..., which later Fathers, including St Gregory Palamas, liked to quote, sums up the single most important aspect of the Transfiguration.  "There is no other place in the entire Bible," as Andreopoulos observes, "where the curtain between the material and the invisible world is completely lifted visually, and there is no other place where the divinity of Christ is witnessed in such a dramatic way"...

The vision of the transfigured Christ, in St Maximus' understanding, implies an internal change in those who seek spiritual knowledge.  There is a progression, he says, from the beginners' stage, in which Christ appears in the form of a servant..., to the advanced stage of those who have climbed the high mountain of prayer, in which Christ appears in the form of God....This manifestation of Christ in his divine nature is not experienced as something external to ourselves.  It is interiorized through the life of faith...

In the Gerontikon, the sayings and stories of the desert Fathers...we find several accounts of monks transfigured with light.  Three of them stand out:  Abba Pambo, "whose face shone like lightening", Abba Sisoes, of whom it was said that "when he was about to die, with the fathers sitting near him, his face shone like the sun," and Abba Silvanus, who was seen "with his face and body shining like an angel".  These texts have been studied with deep insight by Stelios Ramfos, who sees them as presenting us with an image of what it is to be truly human.  Pambo, Sisoes and Silvanus were men whose radiance was the product of inward openness.  In Ramfos' view, Pambo's "if you have a heart, you can be saved," is one of the most important sayings in the Gerontikon.  For the heart in this sense is the spiritual expression of the embodied person.  It is the meeting-place of God within us.  It is where we find freedom of speech before God.  The pure in heart see God, and they become pure in heart through thanksgiving.  It is thanksgiving which enables us to see God, not liberation from the body or the subjugation of the will.  When the heart is filled with thanksgiving, egoism disappears.  And when we are free from egoism, we share in the self-emptying of Christ.  It is only by sharing in the naked humiliated Christ (the kenosis of his divinity) that we can come to share in the glorified Christ (the theosis of his humanity).