But ain't it funny how people demand definitions? Well, that's the place where they can be found. One can't demand definitions from an authority and then refuse said definition from said authority because it doesn't fit your model.
Sure you can, particularly when the definitions given have a ton of nuance not taken into account that don't accurately convey what is being discussed. That's why you often see people say things like "for purposes of this discussion, we are using the specific definition of 'x' to mean 'abc123'"
This is because, as MASH suggested, I use the label atheist to mean I live my life as if no deity exists. I can't know whether or not there is no deity, of course, but since I have no evidence that any exists, certainly not any that anyone has told me about, I live functionally as an atheist and indeed consider myself one. I simply lack the belief.
That is the type of subtle nuance not conveyed by MW dictionary with two short lines.