Spread of Islam
Ottoman Islamic standards of toleration allowed for autonomous "nations" ("millets") in the Empire, under their own personal law and under the rule of their own religious leaders. As a result, vast areas of the Balkans remained mostly Christian during the period of Ottoman domination. In fact, the Eastern Orthodox Churches had a higher position in the Ottoman Empire, mainly because the Patriarch resided in Istanbul and was an officer of the Ottoman Empire. In contrast, Roman Catholics, while tolerated, were suspected of loyalty to a foreign power (the Papacy). It is no surprise that the Roman Catholic areas of Bosnia, Kosovo and northern Albania, ended up with more substantial conversions to Islam. The defeat of the Ottomans in 1699 by the Austrians resulted in their loss of Hungary and present-day Croatia. The remaining Muslim converts in both elected to leave "lands of unbelief" and moved to territory still under the Ottomans. Around this point in time, new European ideas of romantic nationalism started to seep into the Empire, and provided the intellectual foundation for new nationalistic ideologies and the reinforcement of the self-image of many Christian groups as subjugated peoples.
As a rule, the Ottomans did not require followers of Greek Orthodoxy to become Muslims, although many did so in order to avert the socioeconomic hardships of Ottoman rule 
One by one, the Balkan nationalities asserted their independence from the Empire, and frequently the presence of members of the same ethnicity who had converted to Islam presented a problem from the point of view of the now dominant new national ideology, which narrowly defined the nation as members of the local dominant Orthodox Christian denomination. Some Muslims in the Balkans chose to leave, while many others were forcefully expelled to what was left of the Ottoman Empire. This demographic transition can be illustrated by the decrease in the number of mosques in Belgrade, from over 70 in 1750 (before Serbian independence in 1815), to only three in 1850.
Ottoman Empire
By the end of Suleiman's reign, the Empire spanned approximately , extending over three continents. In addition, the Empire became a dominant naval force, controlling much of the Mediterranean Sea. By this time, the Ottoman Empire was a major part of the European political sphere. The Ottomans became involved in multi-continental religious wars when Spain and Portugal were united under the Iberian Union, the Ottomans as holders of the Caliph title, meaning leader of all Muslims worldwide, and Iberians, as leaders of the Christian crusaders, were locked in a worldwide conflict, with zones of operations in the Mediterranean sea and Indian Ocean where Iberians circumnavigated Africa to reach India, and in the way, wage wars upon the Ottomans and it's local Muslim allies and likewise the Iberians passed through newly Christianised Latin-America and had sent expeditions that traversed the Pacific in order to the Christianize formerly Muslim Philippines and use it as a base to further attack the Muslims in the far east. In which case, the Ottomans sent armies to aid its easternmost vassal and territory, the Sultanate of Aceh in Southeast Asia. During 1600s the worldwide conflict between the Ottoman Caliphate and Iberian Union was a stalement since both powers where at similar population, technology and economic levels. Nevertheless, the success of the Ottoman political and military establishment was compared to the Roman Empire, by the likes of Italian scholar Francesco Sansovino and the French political philosopher Jean Bodin.