It’s really difficult for anyone who understands how Christian morality and soteriology works to imagine a place for atheists in Heaven. The ultimate reason is this: You have to want to go to Heaven in order to go to Heaven. Heaven is by definition close to God and is intended to be forever, and being close to God makes it impossible to not believe in Him, and even if you do, it’s difficult to accept spending significant amounts of time in His presence. You couldn’t stand being in His presence during life, why would you enjoy being around Him in death?
Now, nothing about this is to be construed as the idea that atheists go to Hell. Hell has a specific purpose in Christian mythology and atheists don’t fit that purpose.
The third, even more awful possibility is that atheist souls get lost after death. This means you’re not going to Heaven or Hell, but rather you just wander the nonphysical world outside of the kingdoms that God set out for those that believe in Him, never to ever return. All because of what they believed in life.
It’s possible that atheists could decide after death that they were in fact, Christians after all, able and capable of existing in the awesome presence of God. In my imagination though, if you went your entire life purposefully setting your being apart from God, you’re not going to be able to suddenly have that capability after you die, though I wouldn’t want to rule that possibility out either.
The other possibility is that God set out a special part of Heaven for atheists. I think this is probably the most likely option, but I can’t actually see it working for them in the eternal sense. Your existence in the afterlife, without a physical body, is mostly determined by your mind. The spiritual beliefs that are so easy to say aren’t important become really, really important after you die, because belief determines environment in the afterlife. If you believe you shouldn’t have an afterlife, well, you won’t have much of one.
I think honestly the most likely solution is that atheists will immediately reincarnate. But there are angles I haven’t considered, so I’m fairly certain this will change at some point.