In his Beginnings of Judaism, Shaye Cohen devotes a chapter to ancient usages of the meaning of the term ioudaizein. It appears only once in extant classical sources, once in the LXX of Esther, once in Paul, two times in Josephus. Most of the ancient uses are from Greek Christian writers, and “approximately half of the eighty-plus attestations in ancient Christian literature are in passages directly inspired by the usage in Paul” (193).
The term can be used in a political sense (to “judaize” is to lend support to Jews), in a linguistic sense (to speak Hebrew or Aramaic) or culturally (to adopt the customs of the Jews. Cohen argues that “Paul’s letter to the Galatians is the only text in Jewish Greek to use the term joudaizein unambiguously in its cultural sense: to adopt the customs and manners of the Jews” (185).
Among Christian writers, this became a dominant meaning. But the term could spread out to include other actions and attitudes. In its original use, it didn’t mean “to be Jewish” or “to become Jewish,” but in later Christian usage it took on that import. For many Christian writers, “judaizing” described a hermeneutics of the Old Testament, in which the “literal” sense was assumed and the genuine Christological sense ignored.
More tenuously, the term was applied to Christological heretics: “Any Christology that was too ‘low’ . . . was attacked by its opponents as a ‘judaizing’ theology, and its proponents were dubbed ‘judaizers.’ The reason for this is not so much that the Jews reject Jesus . . . but that Judaism as a theological abstraction represents belief in a single undifferentiated God” (191). Hence Arius, who so far as we know didn’t rely at all on Jewish sources, was denounced as a “Jew” and Judaizer.”
Even before we get to medieval and Reformation polemical uses of the word, it’s a term with an interesting career.