Each nation and POP in Victoria II has a religion. Furthermore, each party also has a religious policy which can mean the only religion allowed is the state religion. POPs who are members of the state religion are more likely to support moralism or pluralism while those who are members of another religion are more likely to support atheism or secularism.
POPs which do not have the same religion as the state religion will assimilate into the national religion over time.
Religious policy is a party issue that determines how much the government endorses a particular religion.
Atheism Total separation between church and state. Religion is a private matter.
Secularism Religion is not explicitly part of state policy, but the state tips its hat to traditional religious values.
Pluralism The state religion is actively promoted but other religions are tolerated.
Moralism The church and state are a single body. POPs that belong to a religion other than the state religion face discrimination and are 10% more likely to emigrate.