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.
Arab Revolt
At this stage, Arab nationalism was not yet a mass movement, even in Syria where it was strongest. Many Arabs gave their primary loyalty to their religion or sect, their tribe, or their own particular governments. The ideologies of Ottomanism and Pan-Islamism provided strong competition for Arab nationalism. The vast majority of the Arabs living in the Ottoman Empire were primarily loyal to their own families, clans, and tribes and in terms of broader loyalties, saw themselves as Muslims rather than as Arabs, which in practical terms meant support for the Ottoman Empire, whose sultan also claimed to be the caliph. Traditional Islamic theory preaches the message that all Muslims, regardless of their ethnicity or race, should live together in the "Ummah", where usually been interpreted to mean one state, and for centuries Muslims living under Ottoman rule, whether they be Turks, Kurds, Arabs, Albanians, and so forth all saw themselves as united by a common religion and state. In the Ottoman Empire, Sunni Islam was the state religion, and the sultan-caliph was the supreme religious-political leader, thus making the crown and the faith one and the same. Before 1914, of the ten million or so Arabs living under Ottoman rule, only a mere 350 were involved in various feuding nationalist groups, most of whom wanted only autonomy rather than independence.