I emailed my brother this thread and this is his response which I mostly agree with, posted with his permission.
"I seem to agree with Regolith.
Biblical morality assumes that there is a supernaturally created overarching moral code and I just don't believe that. Things like charity and altruism exist because they are beneficial to the species, not because a "god" willed them to exist.
The people complaining about moral relativism and materialism are missing the point...their arguments are based on their personal feelings about how moral acts make them feel. Which is exactly the point. The other argument relies on saying morality is constant because God said it is, and in the common Christian realm the justification falls on the Bible, which is a circular argument. Once you try to justify a particular morality without relying on arguments that cannot be falsified (and thus are not scientific) I have never heard anyone come up with anything more convincing than simple Darwinism.
That is not to say I think the basic morality championed by the Bible is wrong, most of it is obviously correct...and most of it is absolutely not unique to Christianity. The exact same basic rules exist in every culture simply because without them a civilization cannot exist. If murder and theft were not discouraged, people would never even advance to the tribal stage, and so every existing society by default must have that rule. If everyone stole everything everyone would starve and freeze to death. If everyone murdered everyone would be dead, etc. And so social upbringing re-enforces the ideas that these things must be obeyed, because societies that re-enforce them are more successful.
Which is why the non-central portions of morality fluctuate drastically over time. Sometimes technology or social practice makes them obsolete, like the hygiene laws in the Old Testament. Sometimes societies simply moves past old behavior, like slavery. And sometimes changing traditional behavior results in a net positive, and so is re-enforced...like womens rights and democracy (neither of which would have been encouraged in the Old Testament).
The basic "rules" of morality exist because alternate behavior is destructive. The danger comes when people think the morality that their social group believes in is divinely inspired or somehow perfect. But it never is. Adultery morality is silly to a swinger couple. Homosexual morality is silly to a homosexual. Food and hygiene morality is silly to a culture that understands germ theory. Idolatry morality is silly in a culture that has multiple ethnic groups. If we could create replicator technology theft morality would be obsolete, and if anyone ever discovered true cloning technology murder would not even be a big deal any more.
Perhaps this can also be demonstrated by abnormal psychology. If someone has a defect where every time they kill a person they get a stiffy, guess what, they turn into a serial killer. Many people also exist who simply have no capacity for human empathy. The traditional moral response to this is to label them an inhuman monster, but that is simply because analyzing what is really going on in their heads creates uncomfortable questions for the "moral" person. Many people are unable to accept the fact that they have within themselves the capacity for "monstrous" behavior.
I had a guy at work tell me that World War 2 happened because Satan directly influenced Germany (and Hitler was possibly a demon), because without direct unholy intervention he thought the very idea of genocide was impossible. He totally refused to accept the very idea that a "normal" person could ever be persuaded to commit such heinous acts. But they can, because people have no soul, no divine spark, no built in unbreakable moral system. It is all fluid. It all boils down to chemistry and physics. There is no "you".
Then, by voicing that idea the same "moral" people would be afraid of me. After all, without Christian morality and perhaps without the system of divine reward and punishment to hold my actions in check, wouldn't I just be an untrustworthy, murdering, sex fiend?
But of course I would not. Well, maybe the sex fiend part is true. But it is in my interest to behave in a "moral" fashion. If I help people they help me, if I am kind to others my whole society benefits and so I benefit. If I do not benefit directly, maybe my children will. And to top it off my upbringing still creates guilt about socially destructive behavior, so I still have a emotional force for traditional morality. And if I raised my children without that same emotional reenforcement it would cause problems with negative behavior, so again, it is still in my best interest to teach morality.
But it also means I can pick and choose between why specific items of morality I think are valid. And if I am totally wrong in my choices things will go to hell and the consequences will be dire for myself and my family. Which would also prove my point :)"