Irenaeus of Lyon (c. 130–202 AD) is the father of the theosis doctrine. In Book III, Chapter 19 of Against Heresies, he provides the earliest and most systematic articulation of why God became human: not merely to rescue humanity from sin, but to transform human nature itself. The key formula appears in section 1: “The Word of God, our Lord Jesus Christ, through his transcendent love, became what we are, that he might bring us to be what he himself is.”
This passage is the theological ancestor of Athanasius’s more famous “He became man that we might become God.” But Irenaeus’s version is in some ways more radical: he does not merely say that Christ became human so that humans might be divinized. He says Christ became “what we are” so that we might become “what he himself is.” The directness is striking.
Irenaeus wrote Against Heresies primarily to refute Valentinian Gnosticism. The Gnostics taught that only a spiritual elite possessed the divine spark and could return to the Pleroma. Irenaeus’s counter-argument is that the Incarnation extends the possibility of divinization to all humanity, not through esoteric knowledge but through the union of divine and human nature in Christ. The original Greek of Book III is largely lost; the text survives in a Latin translation. The Greek fragments that survive are noted where relevant.
Propter hoc enim Verbum Dei homo, et qui Filius Dei est Filius hominis factus est, commixtus Verbo Dei, ut adoptionem percipiens fiat filius Dei. Non enim poteramus aliter incorruptelam et immortalitatem percipere, nisi aduniti fuissemus incorruptelae et immortalitati. Quemadmodum autem aduniri possemus incorruptelae et immortalitati, nisi prius incorruptela et immortalitas facta fuisset id quod et nos, ut absorberetur quod erat corruptibile ab incorruptela, et quod erat mortale ab immortalitate, ut adoptionem filiorum perciperemus?
For this is why the Word of God is man, and this is why the Son of God became the Son of Man: so that man, having been taken into the Word and receiving adoption, might become a son of God. For we could in no other way have attained to incorruptibility and immortality, unless we had been united to incorruptibility and immortality. But how could we be united to incorruptibility and immortality, unless first incorruptibility and immortality had become what we are, so that the corruptible might be swallowed up by incorruptibility, and the mortal by immortality, so that we might receive the adoption as sons?
Verbum Dei, Dominus noster Iesus Christus, qui propter immensam suam dilectionem factus est quod sumus nos, uti nos perficeret esse quod est ipse.
For the Word of God, our Lord Jesus Christ, who through his transcendent love became what we are, that he might bring us to be even what he is himself.
Si enim non homo factus esset, non potuisset ut homo recipi. Et si non conditionem hominis recepisset, non potuisset, homo exsistens, quod adversum est homini vincere. Et quod vicit, iuste vicit; et quod dedit, ut Deus dedit, liberaliter et per gratiam. Rursus nisi homo Deo coniunctus fuisset, non potuisset particeps fieri incorruptelae. Oportebat enim Mediatorem Dei et hominum, per suam ad utrosque domesticitatem, in amicitiam et concordiam utrosque reducere, et Deo quidem sistere hominem, homini autem notum facere Deum.
For if he was not made man, he could not have been received as a man. And if he did not receive the condition of man, he could not, being man, have overcome that which is hostile to man. And what he overcome he conquered justly; and he also gave what he gave as God, freely and by grace. And again, unless man had been joined to God, he could never have become a partaker of incorruptibility. For it was incumbent upon the Mediator between God and men, by his relationship to both, to bring both to friendship and concord, and to present man to God, and to reveal God to man.