Religion
Main article: Serbian Orthodox Church
Fresco from Visoki Dečani, Visoki Dečani, Serbia, 1200sConversion of the South Slavs from paganism to Christianity began in the 7th century, long before the Great Schism, the split between the Orthodox East and the Roman Catholic West, the Serbs were fully christian 870 by Byzantine Christian Missionairies (Saints) Cyril and Methodius. After the Schism, those who lived under the Orthodox sphere of influence became Orthodox and those who lived under the Catholic sphere of influence became Catholic, thus, Croats were Catholic and Serbs - Orthodox Christian. Later, with the arrival of the Ottoman Empire, many Slavs converted to Islam. Some ethnologists consider that the distinct Serb, Croatian and Bosniak identities were drawn from religion rather than ethnicity.
wikipedia.
btw Mileva Marić was the co-author of the theory of relativity.