A
semantic clarification, which might make this
discussion easier to have:
Guidelines for making
decisions about the advisability of an action are called
ethics.
The valuations that allow one to develop
ethical standards are
morals.
For what it's worth, the
dictionary (and not just
Webster 1913 backs this up.
A
person with morals has abstract
ideas about what is good and bad, such as "I shouldn't increase
entropy" or "I should experience
pleasure." A person with ethics understands how these rules apply to lived experience, e.g., "I shouldn't blow up buildings" or "I should frequent
raves."
In this system, even a pure
hedonist has morals, and if she lives up to her standards, he is ethical as well. Of course, most people who ask if someone is moral are asking if she is moral by
their standards.
At this point I depart from my
neutral linguistic explanation to enter my own suggestion: atheists can be moral because
Judeo-christian morality is, when
generalized, successful morality. It can be viewed as a matter of
game theory or
memetics: some patterns of behavior allow a society to
survive and
prosper; others cause them to fall apart. Properties like mutual
trust,
collective security, kindness to one's
neighbor, and respect for others'
property will allow people to work together and succeed. Thus,
society is perfectly capable of arriving at what most of us consider moral behavior.
Note that this is not necessarily an atheist viewpoint! God often promises his people prosperity and success, and in later writings he says that he loves them as well. What better way to express love and help the people he chose (or later, those who chose him) then by
explicitly giving them the rules that they'd otherwise have to piece together through
trial and error?