If Serbia, Croatia, Bosnia, and Montenegro all became Atheist do you think they'd reunite?

Yugoslavia was a wonderful idea, that superseded petty self interest, that tried to unite the republics as separate entities with a common goal, sharing the ties of culture, language and tradition. Religion was an obstacle, that's why communists very intelligently didn't ban it, but made it a laughable thing, sort of disrespectful to be engaging in for any rational, intelligent man.

However, with the death of Yugoslavia, extreme religion emerged (look up clerofashism) where the priests again emerged as "guardians of morals and the people", supporting war, violence against homosexuals or any liberal values, and trying to invoke the moral authority of a 13th century institution. Problem with Balkan people is that they are much more emotional than rational, and will buy that.

So to answer the question, if Balkan people evolve so much to understand the danger of tradition invoking religion and can overcome the divisions of the past, yeah, they would.

   

I have loved the stars too fondly to be fearful of the night

Our lives, our past and our future are tied to the Sun, the Moon and the stars…
We humans have seen the atoms which constitute all of nature and the forces that sculpted this work…
And we, we who embody the local eyes and ears and thoughts ...and feelings of the cosmos, we have begun at least to wonder about our origins — star stuff contemplating the stars, organized collections of ten billion billion billion atoms, contemplating the evolution of nature, tracing that long path by which it arrived at consciousness here on the planet earth…
Our loyalties are to the species and to the planet. We speak for Earth. Our obligation to survive and flourish is owed not just to ourselves but also to that cosmos, ancient and vast from which we spring!
We are one species. We are star stuff harvesting star light.