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

Religion (and other civic associations) provide an outlet for public opinion that is often critical of the nation-state.

Outlawing religion doesn't work unless there are no other civic institutions such as unions, political parties, and local governments.

Religion is always the way to speak out against the nation-state even if the leaders are chosen by the state because the prayers of groups of men can be done without leaders.