I suppose, to be an atheist. I think I would find it a very difficult way of life.
The hardest part of it, for me, would be this. If I were to truly be an atheist it would behoove me to understand that everything I see around me and that which I sense within, the things like love, beauty, truth, hope, faith, and more, have no connection to anything transcendent or trans rational either in the universe or myself but are simply illusions created in the wiring of my brain, biochemical reactions that billions of years of random mutations have given me for the sole cause of the continued reproduction of my species. Even the fact that I was aware of such things would be no more than a series of evolved electrical impulses.
That would seem, to me, to be an empty kind of life. Be born, do what’s needed to sustain and pass on my genetic material, and then return to nothingness. Just the realization that this is all there is and that nothing of the spirit, the arts, the beauty, the love, of the world is anything more than a highly evolved survival mechanism would seem to empty any sense of “life” from existence.
I presume, of course, that an atheist would beg to differ but I would be interested in discovering on what grounds? Hope, I suppose would be one answer, hope that the random roll of the dice is still happening and that in some future moment the answers will come, the why of all of those things that seem to make us most human being revealed in a way that requires nothing or no one beyond ourselves. Still, that hope sounds a lot like faith, perhaps even religion.