Source: BBC - Religion & Ethics - Atheism
Wikipedia tells us: "Although some atheists tend toward skepticism and secular philosophies such as humanism, naturalism and materialism, there is no system of philosophy which all atheists share, nor does atheism have institutionalized rituals or behaviors". You can also find a lot of good links at Wikipedia.
"Most atheists feel that the Bible is of questionable accuracy, as it was written thousands of years ago by many authors who were recording oral tradition that existed many years before. Thus, any claimed 'truth' in it is of questionable legitimacy. This isn't to say that The Bible has no truth in it; simply that any truth must be examined before being accepted.
Many atheists also feel that because any passage is subject to "interpretation", any claim that a passage 'means' one thing and one thing only is not legitimate.
Note that this feeling tends to extend to other books.
It is also remarkable to many atheists that theists tend to ignore other equally plausible religious books in favor of those of their own religion." This comes from Common Arguments (1997), a part of the The Secular Web.
Some people think however: "Atheists' morals are not absolute. They do not have a set of moral laws from an absolute God by which right and wrong are judged. But, they do have a legal system with a codified set of moral laws. This would be the closest thing to moral absolutes for atheists. However, since the legal system changes (slavery was legal 200 years ago but is not now), the morals in a society can still change." This is a part of the answer on the question: Can Atheists be ethical? on www.carm.org. Take also a look on their "Proofs for the Existence of God" here.
And it is dangerous to be a sinner: "Sinners in the Hands of an Angry God"