I had an Atheist friend growing up in late primary/middle school. His philosophy on religion (general religion) is that it's there to "keep people good". His stated philosophy on this topic was that the purpose of life was to make a name for yourself.
I can't say how I'd live if I "knew" there was no heaven or afterlife. What is knowing? From the time I've been very small, I've always believed that a God existed. Sure I was trained in Christianity, but I didn't always accept the training I was getting from a particular denomination. This didn't stop me believing that God existed.
Further, I cannot "un-know" certain things. When I play/create music, for example. Though not all the time, there are times I feel such an intense love while playing that I end up overwhelmed to tears. All the good things fill my mind at this time. Love of other people, singing with angels. It's so powerful at times I have to stop because I'm likely to interrupt my own concert with emotion.
Just doing a small good thing for someone else and seeing them react positively. Where does that good feeling come from...a vacuum?? I don't believe that.
At the same time, if I go too far with a negative feeling it hurts me. For someone else, even for God. Why does it hurt me? Why are my days made miserable until I let it go?
These are some things that indicate to me, states of heaven and hell. I think my Atheist friend's opinion was purely cynical and intellectual, without much reflection that these kinds of things *mean something*, they indicate a spiritual aspect of humanity. If there is a spiritual aspect, there is a spiritual realm.