Luther, like many other Protestant reformers, invoked negative images of Islam not only to garner support for a war against the Turks but also to attack his opponents. … Luther was quite fond of tarring both Islam and Catholicism with the same brush. He often used the language of the "Antichrist" when referring to both Muhammad and the pope, though eventually he concluded that the title was more deserving of the pope. Still, Muhammad and the pope were two sides of the same coin:

Green, Todd H, The Fear of Islam_ An Intro…, loc. 154