In Book 2, Chapter 5 of The Ascent, St. John describes both the nature of union with God, and his overarching method for achieving it – depriving oneself of all that is not God.
"To understand the nature of this union, one should first know that God sustains every soul and dwells in it substantially, even though it may be that of the greatest sinner in the world. This union between God and creatures always exists. By it he conserves their being so that if the union should end they would immediately be annihilated and cease to exist. Consequently, in discussing union with God we are not discussing the substantial union that always exists, but the soul's union with and transformation in God that does not always exist, except where there is likeness of love. We call it the union of likeness; and the former, the essential or substantial union. The union of likeness is supernatural; the other, natural. The supernatural union exists when God's will and the soul's are in conformity, so that nothing in the one is repugnant to the other. When the soul rids itself completely of what is repugnant and unconformed to the divine will, it rests transformed in God through love... a soul must strip itself of everything pertaining to creatures and of its actions and abilities (of its understanding, satisfaction, and feeling), so that when everything unlike and unconformed to God is cast out, it may receive the likeness of God. And the soul will receive this likeness because nothing contrary to the will of God will be left in it. Thus it will be transformed in God."
Especially in this second quotation, St. John describes what in the Eastern Orthodox tradition would be called theois, or divinization – the soul becoming like (or even becoming) God.
"A soul makes room for God by wiping away all the smudges and smears of creatures, by uniting its will perfectly to God's; for to love is to labor to divest and deprive oneself for God of all that is not God. When this is done the soul will be illumined by and transformed in God. And God will so communicate his supernatural being to the soul that it will appear to be God himself and will posses what God himself possesses. When God grants this supernatural favor to the soul, so great a union is caused that all the things of both God and the soul become one in participant transformation and the soul appears to be God more than a soul. Indeed it is God by participation."