I would not say that ethical behavior is not possible for the atheist or agnostic. It is. A couple of pretty good examples are Bertrand Russell and Jean-Paul Sartre. However, I will have to say that if we take the human lot as a whole, these two men must be seen as exceptions.
Huston Smith'Blessed are the poor in spirit, for they shall see God' suggested to me a heavenly welfare program for the meek. Today that saying reveals an astute insight into egotism, about how those with swollen pride or vanity cannot see anything larger than themselves.
Huston SmithSo always, if we look back, concern for face-to-face morality, and its modern emphasis on justice as well, have historically evolved as religious issues.
Huston Smith