Atheists reject this assertion as preposterous.  But they are left with having to explain why we have a very deeply ingrained sense of right and wrong.  How is it that we can describe the actions of Hitler as evil, if morality has no deeper sourcing than our current attitudes and preferences?  

This argument by it's very nature is not Biblical.  In other words, an atheist does not recognize Scripture as the inspired and revealed Word of God, and therefore the discussion doesn't center on what the Bible tells us about God or ourselves.  In one corner, Christians assert that God is the source of morality as evidenced in His Word.  In the other corner, atheists ignore the Bible, deny God, and offer up their own explanations for how it is that we can meaningfully discuss the meta-ethical concepts of right and wrong.

I chose to examine this essay because it's intelligible and readable.  The author has a great sense of biting humor that makes it easy to read (if not particularly fun for a theist).

The first point that seems interesting is his denial of historical examples of atheist armies (practically his own words).  I'm not sure how he's defining this, but just in the last century alone we've seen examples of how the officially atheist armies of officially atheist governments have perpetrated horrendous barbarities against their own people, their fellow citizens.  The horrors of life for those caught behind the Iron Curtain in Eastern Europe (let alone the former Soviet Union), or those who continue to suffer under the communist regime in China ought to be caution enough to Mr. Carrier that the atrocities of atheism are evident (if not fully accounted for) in our own recent history alone.  Clearly, when freed from the moral constraints of some deeper order than the State, people are capable of awful things.  While Mr. Carrier admits that atheists can be bad later a few paragraphs later, he seems to be offering up a dismissal of large scale abuse while conceding the point that individually speaking, there are a few bad apples in the lot.  

I'm also not clear on Mr. Carrier's assertion that compassion can be proven to be an evolved trait.  I assume that his assertion here is that it can be argued effectively that compassion is helpful in getting along and surviving.  Most of us would recognize that fact.   Relying on the adage that you catch more flies with honey than with vinegar certainly seems to be a far cry from demonstrating some sort of genetic link to compassion, but that seems to be what he's doing here.  

Mr. Carrier's summary of the Christian argument for morality ("You should be moral because otherwise God will send you to hell") is about as oversimplified as you can get.  I am not moral because I'm afraid of hell.  I'm moral because God has demonstrated that His morality is far more reliable and better than any morality I have attempted to construe or have heard from anyone else.  I am moral because God's Word explains the state of my own soul and the world around me and every person in it in a way that is rationally consistent and compelling and unparalleled by any other world view.  If I found that Christian morality functioned contrary to everything I experience in myself and in the world each day, I wouldn't assume that my response would be to suck it up and believe it anyway out of fear of hell.  My response would be to figure out why this is, and whether or not I'm believing the wrong thing.

Like Mr. Carrier, I'm fond of facts.  We may define facts differently (facts are an interpretation of data, and therefore, as near as I can tell, subject to error in that interpretation and extrapolation), but we all ought to like facts.  I consider myself to be reasonably intelligent.  I'm told in Scripture that this should be my expectation -that God has blessed myself and most every other human being with a brain, and therefore I don't assume that God will require me to ignore my brain to believe in Him.  Indeed, I sometimes find I believe best and most compellingly when I'm using my brain.  Go figure.  And if Mr. Carrier can make a she-goat talk by moving around "a few amino acids in a cell nucleus", he ought to be making a huge name for himself in doing so.  I'm no cellular biologist, but I'm pretty sure that this isn't simple.  Or possible.  

Mr. Carrier seems to evince the typical assertion of atheists that it is our singular inability to point at God and say There He is! or to slip Him under a microscope that generates or requires disbelief.  There is no examination of what God claims to be saying, only that the idea of a God who cannot or will not subject Himself to our scrutiny in an empirical fashion cannot be believed in.  Period.  I find that rather curious.  I tend to think (and yes, I'm biased) that if we examine what is being said, we can have a better idea of the reasonableness or not of the source.  But I'm kinda weird like that.

Mr. Carrier asserts that "thanks to the laws of physics everyone does what they most desire."  I tend to think this is again oversimplification.  Equating actions to the strength of a neural signal does not explain bravery or courage or self-sacrifice or many of the other amazing things that human beings do at the least opportune or predictable times.  Rarely do the surviving perpetrators of such acts say "What I most wanted at that moment was to be brave".  Mostly, they talk about not wanting to get killed.  Desperately.  Right before they found themselves leaping out of the relative safety of their foxhole to dodge enemy fire and grenade an enemy machine gun nest.  Or something to that effect.  It seems that often people do things that surprise even themselves, and equating this to "what they most desire" is at best inaccurate or oversimplified, and at worst just plain wrong.

Rather than focus on the fear of hell as a motivating factor, I think it would be far more accurate to talk about Christian moral motivation as the desire to align ourselves as closely as possible with the way things were designed to be.  We live in a broken and jagged world where perfection is not possible by our own means.  Yet we can say that regardless of our ability to make it perfectly so, there is a way that our world ought to be.  To recognize (as every single person does) that things are not  the way they should be, and to seek to live in consistency with how they should be (to the best of our dim ability to understand and perceive this).  

I would agree with the rudimentary maxim (that Mr. Carrier develops for atheists) that "you should be moral because you will be happier as a moral person overall than if you become any other sort of person".  I tend to think that any moral code has this as the underlying assertion.  It must, because there are times when living by the moral code results in our short-term unhappiness and discomfort, and we must have a greater perspective of things that explains why we're unhappy acting morally.  This maxim applies both to the theist and the atheist. It is a result of a moral code, not the impetus for one.  A moral code exists, and we act in accordance with it because of this maxim that we will be happier overall if we do.  

This still misses the mark of the question he started out to deal with.  He hasn't provided a basis for an atheistic moral code (or even addressed whether such an objective thing is even possible by the basic principles of evolutionary theory).  Rather, he's demonstrated that if someone doesn't want to believe in a God, they can still be a decent human being by following basic moral principles.  

But he started out trying to address the issue of whether atheists have a basis for those moral principles.  Is it possible, based on Mr. Carrier's understanding of genetic predestinationalism and survival of the fittest and what-not, that a few years from now we will have demonstrated the morality of pelting small children with grapefruit?  How can he answer that one way or the other?  What if our genomes have a latent circuit that says that in the event of these specific environmental variables, we ought to find pelting small children with grapefruit morally justifiable?  

It sounds silly, but this is the crux of the issue that Mr. Carrier has humorously sidestepped.  If not something outside of ourselves that defines morality in an objective, always true (if often misunderstood or misapplied) fashion, then can the atheist speak of moral right and wrong with any definitiveness?  Or must the atheist speak of moral right and wrong in a limited fashion.  It is wrong to do this at this time.  Given current situations, this is the right thing to do.  I understand that a few years down the line, these two decisions might be completely reversed.  How do we get anything done if morality is subject to change based on criteria that we don't even know or understand?  

I can appreciate Mr. Carrier's distaste for a God who cannot be poked with a stick.  I'd much prefer that sort of substance myself some days.  But the lack of that substance is not in and of itself an adequate rational reason for rejecting it's possibility or reality.  And if you deny the existence of God and the existence of an objective moral code based beyond our own genomes, psychology, or whims, then what basis can you have for talking about morality in any meaningful context?  How can you sentence someone to execution or life imprisonment if you aren't sure that the crime they were convicted of is actually a crime, and will continue to be considered as such?  If you have no basis for assuming that theft will always be considered wrong, if you have no assurance that murder (or specific instances of it) will always be considered morally wrong, how do you maintain a culture and society for very long?