Theosis (Greek: ‘deification’) is the Eastern Orthodox understanding of salvation: humanity’s ultimate destiny is participation in the divine nature (2 Peter 1:4 — ‘partakers of the divine nature’). ‘God became man so that man might become God’ — the formula attributed to Athanasius (based on Irenaeus) captures the whole of Eastern soteriology. Theosis is not the absorption of human personhood into the divine essence (which would be pantheism) but participation in the divine energies (Gregory Palamas’s crucial distinction between God’s essence, which is inaccessible, and God’s energies, through which he is genuinely present and transforming). The entire sacramental, ascetic, and liturgical life of Orthodox Christianity is ordered toward theosis. Western theology — focused on guilt, satisfaction, and forensic justification — largely lost this category; its recovery in Western theology is one of the significant contributions of 20th century ecumenical dialogue.