Short answer. In Eastern Christianity, yes, and not as fringe mysticism but as the meaning of salvation itself. The teaching is theosis, deification: real participation in the divine life (2 Peter 1:4). The guard-rail that keeps it orthodox is Gregory Palamas' distinction between God's essence, forever beyond participation, and God's energies: God's real operations, which creatures genuinely share. Union by grace, never by nature: the deified saint remains a creature.
This is not a fringe teaching
The phrase sits in the New Testament itself: through Christ's promises, believers "become partakers of the divine nature" (2 Peter 1:4). And the boldest formula belongs to no margin-dweller but to Athanasius of Alexandria, the man who fought harder than anyone for the Nicene definition of the full divinity of Christ. In On the Incarnation 54 he states what the tradition has paraphrased ever since: God became human so that humans might become divine. (That wording is the traditional paraphrase, not a verbatim quotation, but it is a fair one.) Notice who is talking. The theologian most jealous of the Creator-creature line is the one who drew this conclusion from the Incarnation, because for him the two claims are one claim: if God really crossed the gap in one direction, the crossing opens in the other. In the Christian East this never went underground. Theosis is the stated goal of the whole sacramental and ascetic life: salvation not as acquittal but as transfiguration.
The guard-rail: essence and energies
Then the obvious objection: if saints really share God's life, hasn't the creature dissolved into the Creator, pantheism with icons? The East's full answer came in the fourteenth century. Gregory Palamas, defending monks who reported seeing in prayer the same uncreated light the disciples saw at the Transfiguration, drew the distinction Orthodoxy has held ever since. God's essence (what God is in himself) remains forever beyond participation, beyond comprehension, beyond every creature's reach, including the deified saint's. God's energies (God's real, uncreated operations, God-as-self-given) are genuinely shared. The distinction is not between God and something less than God: the energies are fully God, but God turned toward us. So both halves of the paradox hold at once, all the way down. The union is real, and the otherness is real. Union by grace, never by nature. Councils at Constantinople (1341-1351) made this the normative teaching of the Orthodox Church: the saint becomes, in the tradition's own daring vocabulary, divine by grace, and remains, forever, a creature.
The window and the sun
The cleanest image is a window flooded with light at the hour the sun finds it. The light streaming through the glass is really the sun's: nothing in the window produces it, and the window cannot keep it if the sun withdraws. And the window is not the sun, will never be the sun, does not become the sun by shining. Yet it genuinely shines. The saint shines with a life that remains an Other's: that is the whole doctrine in one frame. Vladimir Lossky's The Mystical Theology of the Eastern Church (1944/1957) remains the classic modern exposition: deification as the very content of salvation, made safe (not tame) by the essence-energies guard-rail.. One honesty note: this settlement is settled in the East. Western theology has historically been wary of the essence-energies distinction, worried that it compromises God's simplicity; the question remains contested between the communions, even as it is bedrock within Orthodoxy.
Why it matters for the bigger map
Because it falsifies a lazy story. The lazy story says Christianity is forensic to the bone (a courtroom religion of guilt and acquittal) and that any talk of humans participating in divine life must be foreign contraband, smuggled in from the East or the mystics. Theosis is the counter-evidence sitting in plain sight: a full participation grammar preserved inside Christianity, anchored in its own scriptures, formulated by the same theologian who guarded Nicaea's Creator-creature line, and fenced for centuries with conciliar care. The participatory reading of the divine-human relation is not an intruder in this tradition; it is one of its deepest and most carefully guarded currents. (How the flagship uses that fact in its larger argument is the flagship's own reading, held as a wager. The doctrine itself is bedrock Eastern Orthodox theology, and the posture here is the usual one: borrowing a lens from a living tradition, never presuming to represent it.)
Common questions
What does theosis mean?
Theosis means deification: real participation in the divine life (2 Peter 1:4). In Eastern Christianity it is not fringe mysticism but the meaning of salvation itself, the stated goal of the whole sacramental and ascetic life, salvation not as acquittal but as transfiguration. Union is by grace, never by nature, so the deified saint remains a creature.
What is the difference between God's essence and God's energies?
It is the guard-rail Gregory Palamas drew in the fourteenth century. God's essence, what God is in himself, remains forever beyond participation, beyond comprehension, beyond every creature's reach, including the deified saint's. God's energies, God's real, uncreated operations, God-as-self-given, are genuinely shared; the energies are fully God, but God turned toward us, so the union is real and the otherness is real at once.
Is theosis the same as becoming God by nature?
No, and that distinction is the whole point. The saint becomes, in the tradition's own daring vocabulary, divine by grace, and remains forever a creature; the essence stays beyond reach even in union, so the creature never dissolves into the Creator. The image is a window flooded with light: the window genuinely shines, but it is not the sun, will never be the sun, and cannot keep the light if the sun withdraws.
Do all Christians accept the essence-energies distinction?
No. This settlement is settled in the East, made normative by councils at Constantinople (1341-1351). Western theology has historically been wary of the distinction, worried that it compromises God's simplicity, so the question remains contested between the communions even as it is bedrock within Orthodoxy.
They told you the halo was decoration. The East kept the physics: borrowed light, real shining, and a window that never once mistook itself for the sun.
Sources
2 Peter 1:4; Athanasius, On the Incarnation 54 (deification formula cited as the traditional paraphrase); V. Lossky, The Mystical Theology of the Eastern Church (1944/1957).