You should see how religion works here, in ex Yugoslavia. Those who were the biggest communists prior to the war became the biggest believers during our ethno-religious conflict. Now, when you ask around it’s pretty hard to find non-believers. Even when you find someone who doesn’t really believe, they’ll act according to their family religion, because it’s part of their ethnic identity. According to the 2011 census atheists and agnostics make up 1.16% (0.06% are agnostics) of Serbian population plus there’s 1.39% of people who declared their religion unknown and 3.07% who didn’t declare anything.
EDIT: Christians make up 90.59% of pupulation, Muslims 3.10% and the rest 0.05%.
*without data from Kosovo and Metohija, with them our musilm population would go up over 20% and Christians would go to some ~70%