Atheists will not exist - or would be considered crazy. It would be like not believing in elephants, or possibly like being a Holocaust-denier (not wanting to raise politics, just provide an analogy)
Agreed. The closest thing you'll get to an atheist would be something similar to the Athar from Planescape. "Yeah sure, Zeus exists up on Mount Olympos, but he's not a
god. He's just a very powerful being, but that doesn't make him worthy of worship."
Most people will probably not worship one god exclusively, particularly if your gods have the fairly standard 'portfolio' divisions. There's no point praying to Avandra for sun for your crops, or Pelor for a safe journey.
This is emphasised pretty heavily in Eberron. The primary pantheon is called the Sovereign Host, and consists of nine gods who each have influence over a particular aspect of civilization. Most people worship the entire pantheon. The blacksmith would pray to Onatar to watch over his crafting, to Kol Korran to help with his business, and to Dol Dorn to bring his son home safe from the war. There are some people who focus their worship on a particular Sovereign, but they're considered to be strange.
There's also a pantheon of evil (and one CN) gods, called the Dark Six. Legend has it that they were once part of the Sovereign Host, but were kicked out for being bad. Some people pray to them as well in times of need or just because they feel that a particular aspect of them is appropriate at the moment. For example, someone seeking to woo the love of his life might pray to the Fury, who is the god of Passion.
Eberron's gods also do not involve themselves with the world. They might not even exist - they certainly don't seem to be around on any plane people can go to, or anything like that. People don't have direct communion with the gods or anything like that. That leaves room for differing opinions, and heresy, and stuff like that.
The clergy of the Sovereign Host are pretty accepting of varying opinions. When the human civilization has encountered new cultures and discussed religion, their attitude hasn't been "Oh, your war god is wrong, he does not exist. You should worship Dol Dorn instead." It's been more along the lines of "Oh, so you worship a war god called Gru'umsh. That's interesting. We call him Dol Dorn."