Are you saying, that if one is an atheist, they cannot believe in something after death? If so, why?
You might observe that modern Western atheists almost to a man also deny mind-body dualism or the existence of the soul, and regard mind, emotion, will, indeed all the attributes of personhood as epiphenomina of chemical reactions in the brain.
Buddhists are atheists, and don’t deny something after death: rather they are trying to escape from what they think comes after death—rebirth.
Let me place the challenge in your lap. Tell me how atheists would believe in life after death. I’m unaware of any who profess to believe in such a thing, but would be interested, if there are such folks, in hearing what they have to say.