Deification is the ancient theological word used to describe the process by which a Christian becomes more like God.  St. Peter speaks of this process when he writes, "As His divine power has given to us all things that pertain to life and godliness . . . you may be partakers of the divine nature" (2Peter 1:3,4)

What does it mean to partake of the divine nature, and how do we experience this? To give an answer, let us first address what deification is not, and then describe what it is.

What deification is not.

When the Church calls us to pursue godliness, to be more like God, this does not mean that human beings become divine.  We do not become like God in His nature.  That would not only be heresy, it would be impossible.  For we are human, always have been human, and always will be human.  We cannot take on the nature of God.

St. John of Damascus makes a remarkable observation.  The word "God" in the Scriptures refers not to the divine nature or essence, for that is unknowable.  "God" refers rather to the divine energies---the power and grace of God that we can perceive in this world.  The Greek word for God, theos, comes from the verb meaning "run," "sec," or "burn." These are energy words, so to speak, not essence words. 

In John 10:34, Jesus, quoting Psalm 81:6, repeats the passage, "You are gods." The fact that He was speaking to a group of hypocritical religious leaders who were accusing Him of blasphemy makes the meaning doubly clear: Jesus is not using "god" to refer to divine nature.  We are gods in that we bear His image, not His nature.

What deification is.

Deification means we are to become more like God through His grace or divine energies.  In creation, humans were made in the image and likeliness of God (Genesis 1:26) according to human nature.  in other words, humanity by nature is an icon or an image of deity: The divine image is in all humanity.  Through sin, however, this image and likeness marred, and we fell.

When the Son of God assumed our humanity in the womb of the blessed Virgin Mary, the process of our being renewed in God's image and likeness was begun. Thus, those who are joined to Christ, through faith, in Holy Baptism begin in a process of re-creation, being renewed in God's image and likeness. We become, as St. Peter writes, "partakers of the divine nature" (2 Peter 1:4).

Because of the Incarnation of the Son of God, because the fullness of God has inhabited human flesh, being joined to Christ means that it is again possible to experience deification, the fulfillment of our human destiny. That is, through union with Christ, we become by grace what God is by nature---we "become children of God" (John 1:12).  His deity interpenetrates our humanity.

Historically, deification has often been illustrated by the example of a sword in the fire.  A steel sword is thrust into a hot fire until the sword takes on a red glow.  The energy of the fire interpenetrates the sword.  The sword never becomes fire, but it picks up the properties of fire.

By application, the divine energies interpenetrate the human nature of Christ.  When we are joined to Christ, our humanity is interpenetrated with the energies of God through Christ's glorified flesh.  Nourished by the Body and Blood of Christ, we partake of the grace of God---His strength, His righteousness, His love---and are enabled to serve Him and glorify Him. Thus we, being human, are being deified.   

(The Orthodox Study Bible - Page 1692).