The basic idea is, it is the religious community that matters, the adherents.
The concepts that are sacred to the community are the "cosmic forces" (Xanathars).
People have religious experiences, even miracles, and events that many people in the community experience. Still, it is the people of the community and their response to these wondrous experiences who matter. It is the people that player characters will meet. These are the ones who a player Cleric has as contacts. It is the people who players will encounter during an adventure. So it is important to make the people of each community make sense, and be relatable and interesting.
In other words, whether the religion is true or not doesnt matter. The ideas, activities, and goals of the membersof the community matter.
There is evidence for the belief of the community. The difficulty is, communities with conflicting worldviews each have their own wondrous experiences, so there is uncertainty about which community is "correct", and a person needs to decide whether or not to trust the worldview of the community. And again, ultimately it is the people in the community and the relationship with them who matter.