Deification is an enhypostatic* and direct illumination which has no beginning but appears in those worthy as something exceeding their comprehension. It is indeed a mystical union with God, beyond mind and reason in the age when creatures will no longer know corruption. - Saint Maximus the ConfessorDeification is most often expressed as involving the "Uncreated Light."
Fr. Sophrony says,
This Light penetrates us with the power of God, we we become 'without beginning'––not through our origin but by the gift of Grace: life without beginning is communicated to us. And there is no limit to the outpouring of the Father's love: man becomes identical with God––the same by content, no by primordial Self-Being. God will eternally be GOD for the reasonable being." (We Shall See Him as He Is, p172.)It is though our participation in this uncreated light that we become deified, become like Christ. We do not become a god in essence but by Grace and adoption. We are taught that we can never behold, or know the Divine Essence, but when we are filled with this Divine Light we experience His Uncreated Energies. This is a personal communion with God, face to face. Our identity is not assumed into the Divine Essence. And, it is much more than an experience of Light.
Here is how Saint Symeon the New Theologian expresses it in one of his hymns,
He Himself is discovered within me, resplendent inside my wretched heart, enlightening me from all sides with His immortal splendor, shining on all of my members with His rays. Entirely intertwined with me, He embraces me entirely. He gives Himself totally to me, the unworthy one, and I am filled with His love and beauty. I am sated with pleasure and Divine tenderness. I share in the Light. I participate also in the glory. My face shines like that of my beloved and all my members become bearers of Light.
* Fr. John Meyendorff explains the meaning of enhypostatic:
"This divine light cannot be contemplated as a hypostasis, that is, as an independent reality, since strictly speaking it has no essence. It can be contemplated only in a hypostasis, i'e', in a personal locus. Here Palamas has in mind the deified saints who by grace show forth in their whole persons the light that transforms them. But the energies are also "enhypostatic" in respect of the Person (hypostasis) of Christ. The light of tabor does not reveal the divine essence, but the second person of the Trinity.
As well as meaning "what exists in another hypostasis", enhypostatic can also mean "what really exists"' that which is genuine or authentic, e.g. of our real adoption as sons by the grace of the Holy Spirit. The first sense of the word goes back to the christology of Leontius of Byzantium, the second to Mark the Monk.