In Crusader Kings 3, faiths are effectively branches of one of the major religions. Each character in the world, and county on the map, belongs to a faith, which affects their interactions with you as a player character, and many, many other facets of CK3's world.
Each faith is defined by a set of doctrines (more below) that have huge ramifications for those living under it. This can range from the personal, such as whether gluttony is considered a sin, to the geopolitical, such as how costly and easy it is to declare holy wars. Each faith also has a fervor value. This is affected by many events and choices, as well as the size of the faith itself, and determines the likelihood of heresy - other rulers and counties converting to other faiths with the same religion.
Faith is one of the most complex mechanics in CK3, and there's a lot more to explain. That's why we've assembled the following CK3 faith guide, covering everything from formation and tenets, to doctrines, sins, virtues, and converting other cultures..
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