Theosis or deification is typically considered an Eastern Orthodox concept.  Yet it does crop up fairly often in the writings of Western theologians.  One such is Augustine of Hippo.  In his blog Alvin Rapien collates several Augustinian quotes pertaining to theosis and comments on them, prefacing his remarks with a concise description of what theosis is.  One such quote is reproduced below.  Augustine is at pains to stress that theosis is a participation in the life of God and not a transformation of our created essence into the divine essence.

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So also, just as His inferior circumstances, into which He descended to us, were not in every particular exactly the same with our inferior circumstances, in which He found us here; so our superior state, into which we ascend to Him, will not be quite the same with His superior state, in which we are there to find Him.  For we by His grace are to be made the sons of God, whereas He was evermore by nature the Son of God; we, when we are converted, shall cleave to God, though not as His equals; He never turned from God, and remains ever equal to God; we are partakers of eternal life, He is eternal life.