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Yugoslavia was a wonderful idea, that superseded petty self interest, that tried to unite the republics as separate entities with a common goal, sharing the ties of culture, language and tradition.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.

The Demon Haunted World

There is much that science doesn’t understand, many mysteries still to be resolved. In a Universe tens of billions of light years across and some ten or fifteen billion years old, this may be the case forever. We are constantly stumbling on surprises. Yet some New Age and religious writers assert that scientists believe that “what they find is all there is.” Scientists may reject mystic revelations for which there is no evidence except somebody’s say-so, but they hardly believe their knowledge of Nature to be complete.

Science is far from a perfect instrument of knowledge. It’s just the best we have. In this respect, as in many others, it’s like democracy. Science by itself cannot advocate courses of human action, but it can certainly illuminate the possible consequences of alternative courses of action.

The scientific way of thinking is at once imaginative and disciplined. This is central to its success. Science invites us to let the facts in, even when they don’t conform to our preconceptions. It counsels us to carry alternative hypotheses in our heads and see which best fit the facts. It urges on us a delicate balance between no-holds-barred openness to new ideas, however heretical, and the most rigorous skeptical scrutiny of everything new ideas and established wisdom. This kind of thinking is also an essential tool for a democracy in an age of change.