When people believe in the same god, it is possible that they found church*. And most churches have some clerics of various degree. But most religions have some clerics, even if they are not organised rigorously.
These clerics may say how to believe in their god, or even try to explain god's words. I like Pratchett's sentence about belief from the book Small Gods, which I read in the Czech language, so I am not sure if I can translate it fully correctly:
Belief, he says. Belief shifts. People start out believing in the god and end up believing in the structure - and god dies because nobody believes in him.
All that I wrote above is for gods that are only in the heads of people.
But how could clerics work in a world where gods walk among people (like The Visitor and The Nature from my world)?
*: for example eastern religions are mostly more philosophical way than classical religion. But I don't know much about them.