Whether Christianity is a Avoda Zara for non-Jews is a machlokes Rishonim. The Baalei HaTosafos (and other Ashkenazi Rishonim) held it was not Avoda Zara for non-Jews and the Rambam (who had much less contact with Christians) held it was Avoda Zara. The Meiri who was in between Sephard and Ashkenaz held that it was not Avoda Zara.
That is not Avoda Zara is the dominant position among Ashkenazim.