"How do atheists know right from wrong?" is a question sometimes asked by theists.
In many ways it is the fact that there isn't a god that guides me morally. Our shared mortality, the fact that we are all in this startlingly brief, mysterious, profoundly painful, incredibly complex and utterly beautiful existence together, and that this is the only chance that each of us, each a completely unique being and consciousness, will ever have to exist, is to me what makes life precious and worthy of protecting. For me, understanding the brevity and uniqueness of each life makes it impossible for me not to believe that treating each one carefully is as inherently right as is possible.
Compassion is my religion, not because it has been commanded of me but because I've suffered and I've felt its stain on the precious gem of what will be my brief time here. And if I ache for the chance for this fleeting, singular existence of mine to be as good, as rich and possible to savor as it can be, then it is ultimate human virtuousness to allow the same for other people. If acceptance and love is the most singularly craved experience of humanity, then learning to give it is one of the most meaningful missions one can accept.
Just as many serious theists feel that their journey to live by their beliefs is lifelong work, it is a lifelong journey to learn to fully follow and live by the lead of compassion. It can be paradoxically difficult to figure out how to honor the equally valid hopes of oneself and the ones around them. I have failed, I do fail, I will fail, sometimes and many times - though I hope less as I grow. But despite that, and the fact that not all humans are kind or good, or wish kindness in return, I find it hard to imagine that when the end is reached by any person who strived to let an utter awe and appreciation of both the despair and gorgeousness of life guide their way, that they will feel regret.
That is how I know right from wrong, and why, if you are feel differently, I love you no less.