I don't think that you can generalize with any accuracy on what an atheist believes beyond their unbelief in a supreme being. My mother, grandfather, sister and one of my two fathers were atheists. I was raised an atheist and remained so until my early teens when I became an agnostic. No atheists in foxholes type of thing. In my thirties I became a believer and have vacillated between agnosticism and belief since.
Not without investigation but with a great deal of research and study. I am the only one of the fore mentioned family members who came to believe. My sister is 70 and still an atheist. The other family members went to their grave unbelievers. At present I am a believer for whatever that is worth.