Basicaly Serbs are concidered Eastern Ortodox, on same way as English are Anglican, Irish are Catholics etc.
But, There are (small numers) of Serbs, Cathokics, Muslims even Baptists (in my hometown we have baptist, Adventist and Evangelist church) on same manner, Anglish can be catholic and Irash can be protestant.
As for me, My father is Half Hungarian mother Serbian, and Im Serbian.
Basicly, you are what you fell and act like.
Many of the older generations of "Serbs" --from which I and many American-born Serbs come from --never had "a country" to completely identify with, unless they were from Serbia.
Although my heritage comes from today's Montenegro, where my father was born, Boka Kotorska, was then part of the Austro-Hungarian Empire, not part of Montenegro. For many Serbs from Lika, Bosnia-Hercegovina and Dalmatia, the story was the same -- they weren't Serbs based on geography, they were Serbs based on religion. And in many of the lands I just mentioned, there were no fence-sitters about religion because someone was always trying to convert you or kill you for what you were.
Today, post-communism, you call yourself what you feel like being sans the religious tie. And that is something most American-born Serbs can't fathom about Serbs born in Yugo. Because, based on that logic, I am not "a Serb" at all, but just "an American" in spite of celebrating all the Serbian religious rites and cultural traditions longer and more faithfully than most Yugo-born Serbs have -- or have even been able to do.
But you know, if I let myself think that way, then I'd be out swimming and playing tennis instead of devoting so much of my time trying to help "my people", and that would mean what I have done all of these years is a waste of my time and energy -- and that I don't believe for a second.