Even if there wasnt Religion we would be fighting each other. All wars are made for economic reasons (resources). Example Crusaders conquered Holy Land becouse of important and rich trading cities and routes. They left Muslims to live in their crusader kingdoms, religion was just extra justification. Russians also conquered Poland etc. for economic reasons.

We Slavs will be attacking each other no matter what religion or non-religion. Sadly thats reality. It has been so since antiquity.