According to Christianity (and a number of other religions), no one is good enough for heaven. We have to be perfect, and no one is perfect. So, if Christianity is true, one cannot be a 'good atheist' and hope to get an eternal reward, i.e. being a 'good atheist' is not a loophole that works in both worldviews.
Hence, if God exists and there is eternal punishment, that is a little bit scary.