The Ascension of our Lord Jesus Christ completes cycle of salvation in which God became human in the incarnation of the Word (John 1: ) and then the incarnate Word ascended bodily into heaven. Thus all that divided humanity from divinity came to an end (see my post The Ascension: No Barrier to Heaven Ever Again). God who always wished to dwell with and in us humans, whom God created in His own image and likeness, dwells with us in the incarnation and brings us to dwell with God in the ascension. Salvation is thus by definition the elimination of all barriers to God’s unity with us and the establishment of this eternal communion between humanity and divinity. This definition of salvation was expressed in various ways from the earliest days of Christianity. Norman Russell in his book, FELLOW WORKERS WITH GOD: ORTHODOX THINKING ON THEOSIS (pp 38-39) offers a collection of quotes from early church fathers which repeat this truth.
The Son of God ‘became what we are in order to make us what he is himself’ (Irenaeus, Against Heresies, 5, pref.).
‘The Word of God became man so that you too may learn from a man how it is even possible for a man to become a god’ (Clement of Alexandria, Exhortation to the Greeks 1.8.4).
‘He became human that we might become divine’ (Athanasius, On the Incarnation 54).
‘He gave us divinity, we gave him humanity’ (Ephrem, Hymns on Faith 5.7).
‘Let us become as Christ is, since Christ became as we are; let us become gods for his sake, since he became man for our sake’ (Gregory of Nazianzus, Oration 1.5)
The Word became incarnate ‘so that by becoming as we are, he might make us as he is’ (Gregory of Nyssa, Refutations 11).
‘The Son of God became the Son of Man that he might make the sons of men sons of God’ (Augustine, Mainz sermons 13.1).
‘He became like us, a human being, that we might become like him, I mean gods and sons. On the one hand he accepts what belongs to us, taking it to himself as his own, and on the other he gives us in exchange what belongs to him’ (Cyril of Alexandria, Commentary on John 12:1)
‘God and man are paradigms of one another, that as much as God is humanized to man through love for mankind, so much has man been able to deify himself to God through love’ (Maximus the Confessor, Amgibua, 10).