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.
Ottomanism
Ottomanism was a form of nationalism, likely inspired and created as a reaction to European ideas of nationalism and the growing Western involvement in the Ottoman Empire. Following the Tanzimat reforms, Ottomanism developed of a need to bring the Empire together. The Ottomans feared the growing threat the Europeans posed, especially after events like the Treaty of Balta Liman that allowed for British merchants in the Empire to be taxed equally to the locals, and the growing concern of the Great Powers over the treatment of Christians and Jews within the Empire. The Ottomans thought that if they could unite the Empire fully under one state entity, then they would be stronger and the Europeans would have a harder time encroaching on Ottoman territory, as well as Ottoman people. Previously, the empire was vastly split into many small communities that mostly governed themselves. The Sultan oversaw these communities, however most adhered to their own laws and beliefs. This was a reason the Ottoman Empire was so successful, because the Sultan didn't force any major changes on populations as he conquered them. Though because of nationalism, the rise of nation-states with shared senses of identities began to rise in Europe, most notably with the Greek War of Independence which also started affecting the Ottoman Empire. From these instances Ottomanism was developed as a social and political response, and a hope to help save the Empire from its downfall.