HERE's a question: What about druids? They worship nature oftentimes, rather than even nature gods. Might they be atheists?
Really good question.
I don't have druids per se in my campaign, but I do have shamans that serve very much the same role. The divide between the shamans and the clerics is very much reflective of the divide between the orthodox view of the gods as a higher order of being deserving of pious fealty and humble acceptance, and the view that gods are just another sort of sentient being towards which one may have a more complex relationship and which deserves no special consideration. Shamanism is widely persecuted in many areas because of the often legitimate fear that it is associated with diabolism (summoning and making pacts with fiends), but there are areas where clerics are persecuted for fear that they will bring the gods, and with them subserviance, civilization, and/or cultural change.
I would imagine stock druids, where they are not simply specialty priests of some diety (as is sometimes assumed), would have much the same rivilry with priests. At some level, one is tempted to see this rivilry in purely cynical terms; both sides are fighting for adherents who will turn to them for guidance, leadership, and blessings and whose gratitude they can then draw from.
Heretical groups which oppose the gods often rely on Shamanism for divine power and healing, as there is almost always some lesser spirit out there who has a grudge against a greater one and would happily pull down the gods as well. Unfortunately these are often rogue servitors, fiends, spirits of strife and discord and other quite unsavory spirits, which tends to mean the heretics are usually (but not always!) a pretty nasty bunch who earn their evil reputation.
Note also that this is one area in which my campaign differs slightly from default Gygaxian assumptions, as there is almost no such thing as 'demon worship' per se. While fiend lords of various sorts exist in some form, there is pretty much nothing like sincere servents and worshippers of them. Anyone associating with such things is generally acting out pure cynacism, and heretics might hate all orders of spirits but simply find the lesser sorts easier to bully. The evil gods and their cults tend to serve in the roles Gygax would normally assign to demons, devils, and so forth.
Likewise, I don't have a bifurcated cosmology with monotheistic inspired mythology on one hand running parallel to a polytheistic mythology. It's all polytheistic and animistic. The words angel or demon are not generally used as nouns, but when used at all are descriptors meaning basically good and evil. Those things generally called fiends are simply evil spirits which are, on the sliding scale of godhood, weaker than the ones commonly accepted as gods. I generally have no need of use demons save when I repurpose the stats as a spirit of some vice or as the messenger of some evil deity, and there is no assumption of a native heirarchy controlling the outerplanes somewhat independently of the gods. The same is basically true of the celestials, who show up occasionally but again repurposed as a spirit of something nice or as a messenger or servant of a greater power. (The slaad are an exception, but the reason for this exception is a campaign level secret.) Because the gods are numerous and active, and the PC's assumed to be important, you are just about as likely to ever meet a god in my campaign as anything resembling D&D's normal 'demons as monsters'. On the other hands, generic spirits of stones, trees, fields, streams, diseases, etc. are common as insects. Literally
every tree is presumed to have a dryad or something of the sort associated with it, and there is a stone spirit in literally every rock, and a building spirit associated with every building. Shamans therefore do not want for things to bargain with and/or command.