Without atheists

without god, atheists wouldn't exist

This meme was made to mock organized religion. They are definitely sure that god exists but even if there are no atheists that's only true for the subjective perception. The cowboy is the atheist here getting worried by Buzz's dumb claim.

But you only get that this meme actually makes fun of religious certitude claims if you interpret it well.

Yeah, you're probably right. It's the readers' interpretation that's the problem.