The first instance of the Ottomans having to rule a large number of Christians was after the conquest of Constantinople by Sultan Mehmed II in 1453. Constantinople had historically been the center of the Orthodox Christian world, and still had a large Christian population. As the empire grew into Europe, more and more non-Muslims came under Ottoman authority. For example, in the 1530s, over 80% of the population in Ottoman Europe was not Muslim. In order to deal with these new Ottoman subjects, Mehmed instituted a new system, later called the millet system. Hope this helps!
