TBH, from my experience Andy, I think that, if there are pople who seek eternal  reward they will be more likely to be Christians than Atheists ...

so you disregard Muslims, Hindus, Buddhists, Animists, Pagans, etc.     or do they fit in the 'Atheist' category for you 

I'm pretty sure most Christians don't do it because they believe God loves them.  Some might, but not many.   

I think the difference for me between Christians and the other religious groups I've mentioned above is that only one of these faiths doesn't require someone to act in such a way (the giving of alms, the earning of karma, etc) in order to achieve 'salvation'.