The adoption of paganus by Latin Christians as an all-embracing, pejorative term for polytheists represents an unforeseen and singularly long-lasting victory, within a religious group, of a word of Latin slang originally devoid of religious meaning. The evolution occurred only in the Latin west, and in connection with the Latin church. Elsewhere, "Hellene " or "gentile" (ethnikos) remained the word for "pagan"; and paganos continued as a purely secular term, with overtones of the inferior and the commonplace.
Peter Brown, in Glen Warren Bowersock, Peter Robert Lamont Brown, Oleg Grabar, eds., Late Antiquity: a guide to the postclassical world, 1999, s.v. "Pagan".
via Wikipedia on Paganism