The Ninety-five Theses, published by Martin Luther in 1517, had sparked the Reformation in the German lands and an increasing number of princes turned Protestant. After the Great Peasants' Revolt was suppressed, the 1530 Diet was convoked to calm rising tensions over Protestantism, especially due to fears of the Ottoman advance; the forces of Sultan Suleiman I had almost taken the Habsburg residence Vienna in 1529 and Emperor Charles V wanted Christianity to unite against the invasions. After the 1521 Diet of Worms had imposed an Imperial ban on Martin Luther and his tracts, problems of enforcement emerged, as Charles' wars against France and commitments in the rest of his empire prevented him from focusing on German religious problems.