In a nation-state, what role does shared religion play?

A nation-state is a state that is based on one nationality. Often, people of the same nationality share their religion too, and sometimes it also serves to differentiate themselves from other nations, such as in the case of the Bosnians, Serbians and Croats.

A shared religion can be something that the majority of the population shares.

It taught people how to live a good life, but not what to believe.