This critical volume focuses on the issue of continuity and discontinuity ofthe Christian concept of theosis, or deification, in the intellectual history of ideas. It addresses the origin, development, and function of theosis from its antecedents in ancient Greek philosophy to its nuanced use in contemporary theological thought. Often seen as a heresy in the Protestant West, the revival of interest in deification in both lay and scholastic circles heralds a return to foundational understandings of salvation in the Christian church before the divisions of East and West, Catholic and Protestant. The five sections of this work, written by scholars from the Eastern Orthodox, Roman Catholic, and Protestant traditions, introduce and summarize the nature and function of deification and then lead the reader through four general historical periods of development: Greek and New Testament, Patristic, Medieval and Reformation, and Modern thought. This multi-author work accomplishes what no single author could: it treats the various visions and trajectories of deification that have emerged over the span of a millennium in the various Christian traditions, resulting in a sweeping yet thorough and distinctive contribution to scholarly and informed lay discussion of theosis.