The challenge is, by whatever means necessary, have Islamic nations controlling roughly the territories held by Christian nations in 1500 (Iberian Peninsula, France, British Isles, Germany, Hungary, Italy, Scandinavia, Western Russia, Poland, Ect.) and Christian nations controlling the territories of Islam in 1500 (Essentially the Middle east as far east as India, north africa, the Balkans, Arabia, Eastern Russia, Ect.). The deadline by which the changes should be complete is 1500, and the sect of the areas switched doesn't matter (as in, for Christian Orthodox, Catholic, Protestant, Nestorian, Coptic, and others are all good, and for islam Shia, Sunni, of Sufi are all fine). Bonus points if you can keep western cultures such as French and English alive after Islamification and Arabain and Turkish culture in tact in the Christianized areas.