In Against Heresies 3.19.1, Irenaeus taught a form of theosis:
1. But again, those who assert that He
was simply a mere man, begotten by Joseph, remaining in the bondage of the old
disobedience, are in a state of death; having been not as yet joined to the
Word of God the Father, nor receiving liberty through the Son, as He does
Himself declare: “If the Son shall make you free, ye shall be free indeed.”
But, being ignorant of Him who from the Virgin is Emmanuel, they are deprived
of His gift, which is eternal life; and not receiving the incorruptible Word,
they remain in mortal flesh, and are debtors to death, not obtaining the
antidote of life. To whom the Word says, mentioning His own gift of grace: “I
said, Ye are all the sons of the Highest, and gods; but ye shall die like men.”
He speaks undoubtedly these words to those who have not received the gift of
adoption, but who despise the incarnation of the pure generation of the Word of
God, defraud human nature of promotion into God, and prove themselves
ungrateful to the Word of God, who became flesh for them. For it was for this
end that the Word of God was made man, and He who was the Son of God became the
Son of man, that man, having been taken into the Word, and receiving the
adoption, might become the son of God. For by no other means could we have
attained to incorruptibility and immortality, unless we had been united to
incorruptibility and immortality. But how could we be joined to
incorruptibility and immortality, unless, first, incorruptibility and immortality
had become that which we also are, so that the corruptible might be swallowed
up by incorruptibility, and the mortal by immortality, that we might receive
the adoption of sons?
Here
is the relevant portion from Migne, PG 7/1:939 (see the text in bold above):
