The locus classicus is Athanasius, De Incarnatione 54.3 (c. 318): 'He became human that we might become god.' Second Peter 1:4, 'partakers of the divine nature', supplies the scriptural anchor. Gregory Palamas systematized the mechanics in the 14th-century hesychast controversy: humans participate in God's uncreated energies, never his essence, a distinction defended at the Constantinople councils of 1341-1351 and preserved in the Triads and the Philokalia.
Western Christianity largely filed theosis as an Eastern eccentricity until the 20th-century ressourcement revived it; modern Orthodox writers like Kallistos Ware made it the tradition's calling card. To many Protestants and Catholics it still sounds like blasphemy rather than dogma.
Theosis insists the transformation is real but cooperative: grace does not overwrite the person, it is worked out in synergy, prayer, asceticism, sacrament. The hesychast method, the Jesus Prayer and the guarding of the heart, is the engineering manual, which makes theosis Christianity's most practice-heavy soteriology.
If salvation is deification, what was the church service for?