The world of Faith is home to a great many beliefs, each of which works its own effects on the world and has adherents who directly support it (practitioners, priests, etc).
The story of the world is in large part the story of the evolution of ideas, which can encompass magic and technology religion, trade, political ideas and cultures. The ideas which are more readily able to survive, thrive and spread can eventually come to dominate the world, displacing other ideas which are mutually exclusive but inferior.
Many ideas and practices are not exclusive and so these may all be present in the same region. For example, nation and religion, religion and spellworking practices, practices and cultures, culture and language, language and nation. Also because these ideas often are not dependent on one another, each of them spreads to a different extent, and so going from one distinct region to another one would see a gradual change in the ideas that are different between them, and not a sudden one.
Invasions, missions (missionaries) and trade are the primary means by which ideas may spread, which means that ideas that strengthen the reigning political power, ideas that are effective at getting people to ‘spread the word’, and ideas that require or facilitate trade, will spread more readily.
The crossroads of the world will see the greatest conflux of ideas since ideas must pass through these locations to reach the lands beyond. Major crossroads include Whirla and the Near East (West Estia). By contrast, ideas will often not reach distant, remote lands, and so these may have simpler faiths.
For example, one spelltracing culture has the following ideas in tandem: religion: theism > pantheistic, animism (souls), souls + gems > spiritite, gems + staves -> gem staves, secular magic school, spellriting > spelltracing, nationalism. In addition there may be practitioners of other faiths in this territory, each of which carry their own ideas, provided those other faiths don’t contradict the dominant one. They too would be loyal to the nation they’re in, and to their ethnicity; and so each nation and each ethnicity would have a collection of faiths comparable to their neighbors that would allow them to resist invasion. Those that hold inferior faiths would be overwhelmed, and the faiths of the aggressors would spread into their place.