Religion in Victoria 3 is a feature of countries and pops. Every country has a single state religion (unlike primary cultures, of which it can have several). State religion is fixed for each country, and can only change via events (for example, Japan has the option to change its state religion from Mahayana to Shinto). The Church and State laws determine which pops are discriminated against, based on the relation of the pop's religion to the state religion. A pop discriminated against on the basis of religion will gradually convert to the state religion (or another accepted religion, if it's dominant in their state), the speed of which can be increased by instituting Religious Schools or the Promote National Values decree. Only pops of the state religion can support the Devout interest group.
A religion can have one or more taboos. Pops of cultures for which that religion is a majority are very reluctant to purchase taboo goods if others that can fulfill a given need via good substitution are available, even if the taboo good is comparatively cheaper. Unlike cultural obsessions, taboos are fixed and do not change over the course of the game.