The Transfiguration (metamophoo) of Christ is at the center of the Orthodox understanding of Salvation (theosis).* Western mystical theologians embrace this understanding as well. The manifestation of Jesus' deified humanity to the apostles reveals what will happen to us as well. We need the light of Jesus to shine in our nous/"mind" so that we truly perceive, think, feel and understand. Through His Holy Spirit, the Father shares His divine life in this light. This union has already taken place--the Divine Word and Holy Spirit live in us---yet the process of complete union will last our lifetime and beyond. Until God's light shines in us as it did Jesus on Tabor that day we are incomplete. We are already saved in Jesus, but as the darkness within us testifies, we are not (fully) saved yet!
Let us consider then, this journey into becoming the Light of God, which Paul spells out succinctly in the second letter to the Corinthians: If the Gospel is veiled (kalupto), it is veiled to those who are perishing (apollymi=dying, destroyed, lost, ruined). Jesus is the image of God. So Jesus is the revelation (the apocalypto) or unveiling of God to humans. However some reject Jesus, some will not see God in Him, so they cannot see Him. Paul says such people are dying and dead, lost and ruined---unbelief* is a veil over their eyes and hearts. From whence this veil?
The god of the aeon (age, time period, world) has blinded/darkened the "noema" ("nous"= mind: perception, feeling, thinking, understanding, deciding) of the unbeliever. The darkness and blindness is created by falsehood, the counterfeit wisdom of the world. The darkness is generated by our false desires--the passions--which steal our heart from God and love. The darkness is the fruit of sin and death, it is the environment in which the demonic thrives. Darkness encompasses the powers of this world and its teaching so blindness describes our human perception, our thoughts and feelings, our mental processes and our choices.
Now, let's be clear. People with diminished sight can still get around. Blind people can do amazing things. I saw a bike for blind people at the museum of Science and Industry in Chicago. Physically blind people learn to live around their blindness. And we have learned to live around our spiritual blindness. St. Paul and the church do not say that spiritual blindness totally incapacitate us. They say the spiritual blindness keeps us from seeing Jesus, from knowing God and from the complete union with the Trinity which is the FULLNESS of life.
The teaching on the process of theosis is contained in this verse. This and other verses like it in the New Testament are the source of the Greek Orthodox** church's teaching and the Western*** Church's classical spirituality. We stand at the brink of Lent, the time set aside to renew and reinvigorate our efforts to become what God created us to be---our true selves, aflame with love, enlightened by truth, living in peace and great joy. Let us enthusiastically embrace the way of life!
*I have written about the salvation issue and non-Christians many times. Jesus is bigger than our ideas about Jesus, so, being a Christian does not mean we fully know Jesus. If our faith and knowledge is incomplete and in progress, then it is possible that Justin Martyr is correct when he describes the divine logos at work in the hearts of those who are responding to God without knowing Jesus the man. Jesus is there and the process is the same: dying to self and uniting with God by the Holy Spirit.
** Christopher Veniamin, The Orthodox Understanding of Salvation: ''Theosis" in Scripture and Tradition. (Mount Thabor Publishing, 2016) see especially p89ff "The Light of Tabor: St. John Chrysostom and the Language of Holy Scripture" and p112ff "The Transfiguration of Christ and the Deification of Man in St. Maximus the Confessor"
***For example, the 16th Century writings of St. Teresa of Avila and St. John of the Cross (Dark Night of the Soul). see also the late Medieval "The Cloud of Unknowing." An early church (c-4th Century) source which was extremely influential in the middle ages was (pseudo) Dionysius the Aeropagite http://www.newadvent.org/cathen/05013a.htm