A young man has written to the list a few times. He seems to be "atheist curious" and apparently is being influenced by religion. But rather than blindly accept what he's being told, he is sending the apologetics to us to say "What do you all think of this?"
Whenever I've replied, he's been extremely polite and expressed gratitude. And in his last correspondence, he asked a common question: If atheism offers no beliefs or guidance in life, on what grounds does an atheist tell anyone else they're behaving morally incorrectly?
Here is my reply:
 This is a  very involved question with a lot of angles. I’m going to include some  links, and explain why I believe they are relevant. This is a question  many different people in many different cultures through time have  attempted to address. In the end, as with all such questions, you are  going to have to process the data and try to draw the best conclusion  you can based on your observations and values.
  First of  all, let’s start with the prior base [which he had already agreed to in a previous e-mail], that humans are demonstrably  social animals. You can see we are. We live in societies all around the  globe. Other social animals we observe include lions, wolves, dogs,  and so on—any animal that lives in a community and requires  cooperation, generally, to survive. Lots of animals aren’t social, but  we can see when they are; and humans clearly are.
  This means  we have evolved things like compassion, guilt, concern, and so on. We  have all the individual survival instincts, but also instincts that  cause us to care about others to some degree. People will have these to  different degrees. Some will be so compassionate they won’t hurt a fly. Others will be so uncaring they will be labeled as sociopaths.  Nearly all physical traits, whether they affect our minds or bodies [obviously not intended in the dualistic sense, but in context of a discussion on morality],  will be spread through the population on a bell curve, where there will  be a “normal” range, where most of us fall, but then extremes on either  end. So, we see most people have brown eyes around the world, or  cholesterol that falls in a certain range, or are within a normal weight  range, or have normal intelligence, etc. And there are always people  who fall within more or less “normal” ranges. This diversity is actually  beneficial to us as a species, because  adaptability depends on being able to move the population in different  directions. The “normal” ranges for us now are simply “where we’re at”  currently, but people can, for example, get to be “taller” on average  than they were 200 years back.
  So, we  have these basic sets of normal emotional ranges that encompass our  interactions with other people. But they are very basic. You can see  this in domestic dogs. We are able to train dogs easily because, like  us, they are highly social. So, they have some of the same emotional  ranges we do when it comes to understanding “right” from “wrong”  behaviors. People can easily get a dog to understand good behaviors by  rewarding the dog. And likewise, we can train a dog that  certain things are “wrong”—such as biting people or jumping on the  sofa. If the dog “knows” it can’t jump on the sofa, it will display  behaviors of submission if you catch it on the sofa. So, it may put a  tail down, or whimper or slump—to show you it knows it did what you  don’t want it to do. The dog is socialized, and this is why it is easy for people to train and work with  dogs.
  People are similar. We have basic sets of underlying feelings about  cooperative interactions. Some authors talk about an underlying sense of  “fairness.” You can see this at an early age. If a child possesses a thing it  likes, and you grab it away, the child becomes upset. Nobody likes  to have something they like taken from them. That’s a basic feeling most  of us share. Also, nobody likes pain. And to a high degree, if we’re healthy and well, most  of us prefer living to dying.
  Now, in  reality, there are societies where “fair” includes things that here in  the U.S. we don’t think are fair at all. For example, in some areas of  the globe, if a woman walks down the street unescorted, she might be killed,  and it’s actually sometimes considered correct for people to harm or kill her for that behavior.
  The  question you are asking is: What do we do when we think it’s wrong to  treat a woman this way—but an entire other society thinks it’s OK? How  is that resolved?
  But the  problem is the same within a culture, as well as between cultures. Here in the US, we have  disputes about whether or not many things are OK, or not OK, for people to  do. There are a lot of arguments about whether drug use should be  criminal or whether abortion should be legal. And you probably have seen  or heard people arguing about these things.
  You are absolutely correct that atheism  does not resolve any of this. Atheism  only means you don’t believe a god exists. So, atheism really would not  be the right place to look if you wanted to know about something like  “what is moral action?” For that, you’d want to consult behavioral  psychology or even philosophy. You’d have to do a lot of reading and  thinking to figure out what you think is right and what you think is  best.
  Here are  some links as examples:
  http://www.nytimes.com/2008/01/13/magazine/13Psychology-t.html?_r=2&ref=science 
 http://en.wikipedia.org/wiki/Moral_philosophy   
   http://en.wikipedia.org/wiki/Moral_dilemmas
  For  myself, I tend to think that if I wouldn’t want to be treated badly,  it’s best not to treat others badly. Jesus used the Golden Rule, and a  man named Stephen Covey  used a Platinum Rule. Jesus said it was best to  treat people how you’d like to be treated. Stephen said it’s actually  better to treat the other person how the other person would like to be  treated, since he or she may not like the same things you do. Other societies have used  other versions of this idea, with things like “don’t do things to people  you wouldn’t want done to you.”
  Additionally,  there is a question of how much control we should have over others. If  what you do doesn’t hurt me or cause social harm, should I pass laws to  stop you from doing it? This is at the heart of arguments about things  like gay marriage.
 If you  don’t believe a god is telling you what to do, that means you become  responsible for trying to figure out morality on your own and for coming up  with the best ideas you can about how you ought to treat others.
 In the end, people make the rules for human society. And we must all ask  ourselves how much we want to be involved in that. If there is a vote  for gay marriage in my area, will I vote for it, against it, or do  nothing? That’s what I have to decide for myself. Do I want to help  them? Impede them? Or do nothing and leave it to others to decide?
  And then  we have the question of societies and whether or not they should  interfere with one another. This is also a personal question each of us  is responsible for answering. If a neighboring culture is rounding up  Jews into prison camps, and torturing and killing them—do we care? Do we  intervene? There is a lot of debate and heated argument over things  like this. For a long time, the U.S. hesitated to become involved in WWII. Should  we have done something sooner? Should we have done nothing? That’s a  question each person must answer for him/herself. Do you push your  legislators to get involved? Do you tell them not to get involved? Do you do nothing  and leave it to others to decide?
  What are  your values? What do you want from life and other people? What sort of world do you want to live in? What do you  feel are your obligations toward others? What is your tolerance for  personal suffering, or for the suffering of others?
 These  aren’t easy questions. But religion tries to pretend they are.
 It is very easy to say “God’s will be done…” and leave it other  people to do the work in this world.  
  I know you  did not specifically ask about the following, but I want to offer it,  just as something to consider. And I hope it’s OK.
  Often when  Christians ask something like you just did, they mean something like  this: “I get my morality from god/the Bible; but without those, where  would I get morality?” I know this is not what you said specifically;  but it reminds me of this question in some ways. And there is an  additional dilemma here that many religious people fail to consider.  Long ago a man named Euthyphro had a thought that went like this:
  “Is what is morally good commanded by God because it is  morally good, or is it morally good because it is commanded by God?"
  What he is asking, is whether there is such a thing as  “morality,” or if morality only means “doing whatever god says.”
  The problem comes in with verses in the Bible like these: 
1 Samuel 15:2-3: "Thus says the Lord of hosts, ‘I have noted what Amalek did to Israel in opposing them on the way when they came up out of Egypt. Now go and strike Amalek and devote to destruction all that they have. Do not spare them, but kill both man and woman, and infant, ox and sheep, camel and donkey."
Exodus 21: 20-21: "If a man beats his male or female slave with a rod and the slave dies as a direct result, he must be punished, but he is not to be punished if the slave gets up after a day or two, since the slave is his property.
Leviticus 20:13 "If a man lies with a man as one lies with a woman, both of them have done what is detestable. They must be put to death; their blood will be on their own heads."So, in the Bible, we have Old Testament passages that state clearly that  god told people to go and commit genocide against their  neighbors—even killing infants and animals. Then, we have two passages from the Law of God, one that describes how it’s OK to have a slave and beat  the slave near to death, and another that says we should execute gay  men.
  Obviously today, we would never consider these acts anything  less than barbaric. If a country committed genocide, they would be  globally condemned. If a country sanctioned slavery, we’d condemn that as well.  And in Uganda, where they actually are passing laws to execute gays,  there is an outcry against that law as an atrocity.
 So, the question is, is there anything really wrong  with killing gays, infants, and beating people near to death?   
  If morality is simply “whatever god says,” that means these  things aren’t actually wrong. It means that sometimes it’s right to do these things.  Any Christian  who says “That was the old  testament” is plainly saying  “I agree that sometimes it’s right. When god said it back then, it was  right. I agree it should have been done.”
 Unless they’re willing to  say it was wrong in the Old Testament—even if god said to do it—then  they’re claiming sometimes it’s OK to have slaves and beat them, kill  gay people, and slaughter infants in droves.
  Were these things ever OK to do to other human beings? If a  person answers "yes," then they have no moral compass. They are saying any  action can be moral or immoral, all it takes is for god to say “do it”  to make it “right.”
  If they say that actions are not moral "just because god says  to do them," then the response is that these verses I just used  demonstrate Yahweh tells people to do immoral things. A moral person would want to stop a person from beating another near to death as "property." A moral person would want to stop a person from slaughtering babies out of pure vindictiveness. A moral person wouldn’t ever stand by and let someone kill someone else simply  because they're gay.
 Usually the Christian response is that god knows better, and  when god tells people to do horrible things, there is a greater good at  work. We're told we can’t recognize the larger plan, because we’re just humans, and not  gods. But the problem there is: If you can’t tell a good action from an  evil action, then how do you know it’s good if god says to go kill  babies? It sounds evil—so what makes a person accept it’s good?   
   And it appears to come down to this:
  If god says to do something awful, should you do it?
  And here is my answer:
  If I can’t understand how it’s good, and it seems evil, I  can’t do it. Ultimately I am responsible for my actions. And if I don’t  do this action, at least I can justify to you why I didn’t do it—why I  judged it was evil. But if I blindly trust an authority, even when the  action appears clearly to be evil, how do I know what I’ve done really wasn’t the evil it appeared to be? How  can I justify my actions in that scenario? I can’t. I can only hope the atrocity I  committed wasn’t really the atrocity it seemed.
  And I couldn’t live with that level of irresponsibility. I  need to know what I’m doing and why if someone wants me to do something I cannot justify as moral.
  Again, I hope any of this is helpful.