Theosis, or deification, is the doctrine, central in the Christian East, that the goal of salvation is not merely pardon but participation in the life of God: to be, in the words of 2 Peter 1:4, "partakers of the divine nature," united with God and transformed by him, while never becoming God in his essence. The classic slogan is Athanasius' statement that God "became man that we might become god."
The doctrine turns on a distinction the East draws sharply: between God's essence, which remains forever unknowable and unshareable, and God's energies, his real activity and self-communication, in which the creature can genuinely partake. On this account salvation is a true union with the uncreated God, the believer really shares the divine life, without the pantheist collapse in which the creature would become a piece of the divine essence. Deification is thus offered as the answer to what human beings are ultimately for: not a legal transaction completed and filed, but a transformation without a natural ceiling.