Complex sentence: Alister McGrath, former Professor of Historical Theology at the University of Oxford, claims that the 1st century "Jewish Christians" were fully faithful religious Jews, only differing in their acceptance of Jesus as the Messiah.
Split into simpler sentences: Alister McGrath, former Professor of Historical Theology at the University of Oxford, claims that the 1st century "Jewish Christians" were faithful religious Jews. They only differed from other Jews, in their acceptance of Jesus as the Messiah.