The Eastern Orthodox understanding of salvation is centered on a doctrine known as deification, or theosis.
This teaching means that human salvation rests upon growing in the likeness of God.
We don't strive to become God Himself, but we strive to become a true human person which is the image and likeness of God. Mankind is called to fulfill their created purpose, eternally growing in likeness to God, and in turn becoming truly human.
This process of growth is a cultivation of the existing image of God within the human person and is exhibited through a true union of personal thought, word and deed with the will of God. It is in this sense that St Paul writes in Galatians, "it is no longer I who live, but Christ lives in me (Gal 2: 20)."
We are to become so Christ-like that our own will becomes the will of Christ. Fourth century theologian St Athanasius of Alexandria wrote that, “God became man so that man might become God." The Son of God became what we are in order to raise us up to what He is.
When deified, mankind becomes all that God is except for identity in being, becoming “partakers of the divine nature (2 Peter 1: 3 – 4).” Theosis describes the transformation of the person through the complete transfiguration and sanctification of soul, mind and body.
The participation of humanity in God is not a “static given” but involves the challenge of overcoming the passions, and growing closer to the divine likeness. The attainment of this state of communion is only possible through the grace of God, but nonetheless requires the cooperation of the human freewill in accordance with divine grace.
Through deification, humanity becomes “by grace what God is by nature,” but this also requires that the person cultivate the image of God within themselves as a choice of their own freewill.
The gift of grace and the freedom of mankind working together is a process called synergy.
It is only through God’s will for human salvation and a person’s will to be united to God that deification is made possible.
Becoming united to God is not solely an intellectual pursuit, but requires the sanctification of the whole human person. This process of transformation involves the body as well as the mind, and therefore is necessarily sacramental.
The sacramental nature of deification is made abundantly clear through communion in the Holy Eucharist, when the true Body and Blood of Christ becomes a part of our very substance, thus transforming and deifying our entire humanity.
Mankind is called to become the true image and likeness of God in all aspects of their humanity. This constitutes a working out of the divine love through a human life.
As St Gregory of Nyssa writes, “God is love and the source of love. The creator of our nature has also imparted to us the character of love… If love is absent, all the elements of the image are deformed."
Similarly St Maximus the Confessor says that, “If God is love, he who has love, has God within himself.”
It is this manifestation of the love of God in humanity that ultimately leads to deification.
In the Eastern Orthodox view of salvation, good or bad works do not directly affect a person’s salvation in a transactional way, wherein merit with God is either built up or lost, but rather in the transformation of the person that occurs in response to our actions.
It is this transformation that leads to deification, when one does good works not for personal gain but out of the true unselfish love of Christ.
We do these things because our will has been united with the will of Christ.
Humans do not lose their freewill when becoming united to God but rather conform their personal will to the will of God.
It is possible for humanity to experientially know God and commune with Him.
Humanity can know God through His energies (His presence and interaction with creation) but not His unknowable and transcendent essence. Thus mankind can become fully transfigured in the likeness of God but does not become the essence of God.