Complex sentence: They assert that Christianity and Judaism did not separate as suddenly or as dramatically as sometimes thought and that the idea of two messiahs one suffering the second fulfilling the traditional messianic role was normative to ancient Judaism, in fact predating Jesus.
Split into simpler sentences: They assert that Christianity and Judaism did not separate as suddenly or as dramatically as sometimes thought. And that the idea of two messiahs one suffering the second fulfilling the traditional messianic role was normative to ancient Judaism in fact predating Jesus.