The Introduction surveys the different meanings of theosis and religion that have evolved over the centuries as a way of defining the book’s principal terms. Then after discussing the recent debate about whether theosis is a doctrine or a theological theme, it introduces the subsequent chapters as an exercise in intellectual history. The narrative thread of this history is provided by the different receptions of Dionysius the Areopagite in Eastern and Western Christianity. Theosis emerges as a polyseme, its meaning defined by the different contexts in which it is used.