Here's a question that arose on Yahoo! Answers, and my response
Atheists-How do you know what is right and wrong?
You can't really say "my conscience" because that would suppose something bigger than ourselves telling us what to and not to do and that would reveal another chink in the armor for evolution. Also, since you don't realize that God exists, where do these morals come from?
And here is how I responded:
While I disagree with the idea of Right and Wrong, I will respond to this from the standpoint of differentiating between helpful and harmful.
Human beings understand that there are actions that will be helpful or harmful. It's obvious that we don't want ourselves or our loved ones hurt, and we can extrapolate that this is true for others as well. Therefore, the idea of avoiding actions that can harm others is a commonly held, rational tenet.
Doing what is helpful is a bit trickier, because the ramifications of help can lead to dependence, which is often harmful, or self-righteousness, which is also often harmful. Being helpful requires either a negotiation and mutual agreement regarding the help or a determination that the person cannot negotiate and we are to be helpful without their assent.
Determining that God is the arbiter of right and wrong can actually lead to greater harm. For example, there is little doubt that Gay marriage would be helpful in society: It would lead to more stable relationships, would promote quality health care because of increased insurance coverage, would avoid costly legal maneuvers needed to secure couples' medical and inheritance rights, and would ease anxiety within the couple regarding their legal safety. However, because of citations in the Bible (said to be from God) that homosexuality cannot be condoned, many Christians are preventing this helpful course with nothing to support their objections beyond the statements in the Bible.
This is one example of theistic ethics leading to greater harm. There are countless others that come from all variety of religions, including slavery, subjugation of women, child abuse, murder of witches and other outside-the-faith groups, feudalism, and business restrictions, to name only a few. Each of these harms is (or was) held in place by believers in God's law; ethics gleaned from the rational rule of "do no harm" will prevent these injuries to human life, health & dignity.