The religion that a nation follows and how tolerant it is of other faiths is an important aspect of gameplay in EUIV. The religion of a nation will confer specific benefits, enable different mechanics, and affect diplomatic actions as nations of mutually accepted religions have a better chance of reaching agreements with one another. Religion is also connected to unrest and provinces of non-tolerated religions are more rebellious. The player has some control over religion by having the option to change the state religion, send missionaries to convert heathen or heretic provinces to the state religion, and carry out religious decisions.
Additionally, positive tolerance of heretics and heathens allows those provinces to contribute to religious unity (see above).
Note that there are some national ideas which remove all penalties for having negative tolerance.
American idea 1: Freedom of Religion
Hungarian idea 7: Create the Estates General
Piratical idea 1: Religious Apathy
Tolerance of the true faith
This refers to the state religion. For example, if the state religion is Catholic, Catholic provinces will use this tolerance value. The base value of tolerance of the true faith is +3, and there is no maximum value.
This refers to different religions within the same religious group. For example, if the state religion is Catholic, the religion group is Christian, and provinces that are Protestant, Reformed, Coptic, Orthodox, or Anglican will use this tolerance value. The base value of tolerance of heretics is −2. The possible maximum value is +3.
Tolerance of heretics of ≥ 2 will allow provinces to randomly convert to those heretic religions. The mean time to happen is 5000 months, lowered by Innovative Ideas and having a neighboring province with the new religion while increased by having the theocracy government type. Provinces with the "Religious Zeal" modifier will not receive these events at all.
A province is more likely to convert via event to a particular religion if a neighboring province has that religion. Adjacency is not required for most conversions. However, the following conversions only occur if a neighboring province has the new religion: