A man who has no assured and ever-present belief in the existence of a personal God, or of a future existence with retribution and reward, can have for his rule of life as far as I can see, only to follow those impulses and instincts which seem to him the best ones.
Autobiography of Charles Darwin. As compiled and edited by his granddaughter, Emma Nora (Darwin) Barlow, p.94.
If each individual would do what is best in their own eyes, guess who’s interest each person would consider best. Their own. Darwin’s astute observation can be further delineated by Emile Durkheim‘s that society is the ultimate constraint to mankind’s behavior. With this, the best that a godless mankind can be hoped for is Machiavellian: Everyone will do what is in their own best interest, which may include convincing others that one is doing what is best for others. Note this is not necessarily the same as doing what is best for others as individuals or society at large. If lying and false impressions achieves that, so be it. The net? Society and mankind (and ultimately the individual) loses if their is no perception of supernatural accountability beyond himself. So the answer for the faithless person is to convince everyone else that he or she believes in god and that everyone else should believe in god, for their own sake. I’m afraid there is a lot of that going on out there.
But let me take a deeper track: What about the truth? Does it make sense for a biology to evolve without god in a way that requires a belief in god by its highest form in order to function at its best? The answer to me seems simple: No.