Actually, it was quite common in the Middle Ages for Europeans to consider Islam a Christian heresy. https://books.google.com/books?id=CY-sAgAAQBAJ&pg=PA520
Actually, it was quite common in the Middle Ages for Europeans to consider Islam a Christian heresy. https://books.google.com/books?id=CY-sAgAAQBAJ&pg=PA520
Actually, it was quite common in the Middle Ages for Europeans to consider Islam a Christian heresy. https://books.google.com/books?id=CY-sAgAAQBAJ&pg=PA520
Not just the Middle Ages. More traditional Catholics will still argue that it is. Just a hundred years ago, Belloc in his The Great Heresies characterized Islam as the logical conclusion of Arianism (he said Mohammad started with Catholic/Orthodox teaching and stripped away the Incarnation), and referred to it as the one heresy that originated outside the Church.
Oh, I know that Belloc said that, and that Evangelicals sometimes quote C. S. Lewis to the effect that the only two religions worth considering are Christianity and Hinduism: "I have sometimes told my audience that the only two things really worth considering are Christianity and Hinduism. (Islam is only the greatest of the Christian heresies, Buddhism only the greatest of the Hindu heresies. Real Paganism is dead. All that was best in Judaism and Platonism survives in Christianity.) " http://www.patheos.com/.../2014/08/22/the-worst-of-c-s-lewis (I won't say this is the silliest thing Lewis ever wrote, but it comes close...)