There is a theory, which makes sense to me, that humanity has been evolving morally throughout its history. Tribal units require one sort of ethics, national units another, and a global society needs something else entirely. But each stage helps prepare things for the next.
I saw this, at first, as a proof of God. He must be leading us slowly toward better virtue; away from "an eye for an eye" and toward forgiveness, away from hierarchies and toward equality. That's why the Old Testament is full of things that are morally problematic for me, but slowly things like "I desire mercy and not sacrifice" come in, and eventually Jesus shows up with a much better moral code.
However, I didn't realize at the time that humanity doesn't seem to move from one stage to another entirely because of God's leading. Rather, when trade becomes a more important source of wealth than warfare (for instance), tolerance toward outgroups naturally increases. First, because one is in more contact with outgroups and therefore will be more likely to see them as people worthy of empathy, and second, because peace promotes more opportunities for trade and so aggression should be minimized. So, as in so many things initially taken as proofs of God (like the creation of different species of animals), once one finds natural causes, its value as a proof vanishes. Natural causes don't show God can't have been involved, but they show he doesn't have to have been.
I have strong moral objections to Old Testament life. I don't like patriarchy, I don't like the death penalty, I don't like total war, and so forth. I think it's horrible that a woman could be married to a man against her will and then divorced against her will. Circumcision is (to my mind) nothing more than inflicting pain on babies. Stoning adulterers, gays, idol-worshippers, and so forth is a horrible thing to do. No, I am not interested in that.
Jesus brought everything to a whole new level . . . or so it seems. Some people attribute a lot of his moral innovations to others -- similar ideas are found in Philo of Alexandria, Socrates, Buddha, Confucius, etc. Was the Golden Rule just an idea whose time had come -- were people finally ready to love their enemies? Either way, the Christian way was definitely an improvement.
Still, life was sub-optimal in the Middle Ages, and one of the reasons it wasn't as great as it could have been was that people's morals had some flaws. Just war theory had been described, for instance, but it wasn't followed much. And gender equality, prohibition of slavery, self-government, and human rights weren't even really theorized. I'd like to say that God was slowly leading people to develop these things -- that they are there in seed form in the New Testament and it just took time to "unpack" them -- but in reality the Church did not lead the charge here. All four of the things I listed, which I think of as Good Things, are Enlightenment values which the Church later got on board with because they were so obviously good. But still, there's a strong Catholic argument from tradition against each of those things -- hang around traditionalists long enough, you'll hear it.
It bothers me that some of the moral laws I consider most emphatically true -- the ban on the death penalty and slavery, consent required before and even within marriage, the equality of all human beings, religious freedom -- are recent innovations within the Church. You would think that God would have made sure we got those. It's not like he couldn't have quite easily mentioned somewhere in the Bible that slavery was bad, instead of telling slaves to submit to their masters or suggesting that beating slaves was helpful. If he had wanted us to know women were equal in dignity to men, why didn't he ever say so? Instead he set up a church where only men can be priests and included some helpful verses about how women were created last and were responsible for sin, so we should wear veils. I'm not complaining at the moment -- I'm saying, if women are in fact equal in dignity, and if this was something morally important for us to know, you would have expected he'd slip it in there someplace.
I worried about all this for a long time, because after all morals do vary according to time and place, and perhaps my own conscience can't be trusted because it too is a product of a time and place. There are certainly people around today who think that women aren't equal, that slavery might be part of a natural order, that the death penalty is required by retributive justice, and that religious freedom should never be granted because error has no rights. There are even people who go straight back to the Old Testament and pull verses out of there to prove we should still be stoning gays to death. On what grounds, without appealing to God (because God is notoriously hard to understand) can I say that my moral sense is right and theirs is wrong?
Well, the answer is sort of obvious, but I didn't see it until it was pointed out in a book called The Moral Landscape which I read recently. (I can't actually recommend it; it had almost nothing noteworthy in it except the part I'm going to tell you here.) Taken on a group level, the moral standards of a group of people are either good for them or bad for them. Obviously on an individual level this doesn't work; doing what is good for me personally might be bad for others and would be an immoral thing to do. But on a group level, a good moral standard is one that has good results for everyone. Everyone is better off -- happier, healthier, living longer -- when everyone acts morally. If murder is off the table, we all get to not be murdered, so everyone wins.
Some moral laws might not be obvious in the way they help everyone. For instance, the Celts had a law that a woman couldn't be married until she was twenty. That was surely inconvenient, because 18-year-old girls are both pretty and prone to falling in love, but it had a hidden benefit, because getting pregnant too young is very bad for a woman's body. They believed, correctly as it happened, that the children would be healthier if a couple waited till the woman was twenty. It was a good moral law for them to do that.
So I could take any moral law there is and ask, is it good for people to do that, or is it harmful? Does it result in happiness for a few and misery and anguish for the rest? Does it result in happiness for ingroup members and conquest and death for the rest? What we want is a moral code that results in more happiness, health, and life for everybody. In Catholic language, the moral law will serve the common good. What is moral is good for us. Even if you assume it is revealed by God, what God reveals to be good is not arbitrary, but based on what he in his omniscience knows is good for us.
Taking this as the standard, it becomes much easier to say that some moral laws are objectively better than others. While it's true that it may not always be possible to say if a specific action is harmful until long after the fact -- I mean, just watch the debate about whether or not gay marriage will destroy society -- it's pretty easy to tell, with the perspective of history, that some moral laws are deficient and others are better.
Does circumcision have a benefit that outweighs the harm it does? Science can tell us the answer. I've done a lot of study and I think the only way to return a "yes" here is to assume that it must because God commanded it. Ditto the death penalty. Slavery and the oppression of women may have had a slight benefit for slave owners and men, but it had a huge cost for slaves and women, so I think we can fairly say it was bad too.
So here's what I'm getting here: there are two ways of coming to a moral code, revelation and human consideration of the common good. Catholic teaching, of course, comes from both -- things we've learned from Scripture, plus things patristic, medieval, and modern philosophers and theologians have worked out. The latter category is a bit of a tossup because the credit for just-war theory, self-defense, women's rights, best forms of government, and so forth has to be shared between God and humans. These Catholic moralists might cite Scripture or tradition as a support for their views, but they also consider science (according to what they had at the time) and common sense. Even you think Catholic moral teaching is excellent, as I generally do, you have to acknowledge that it is in part a human creation.
And when I try to develop a moral sense based on reason and the common good, and then try to hold God's actions to a similar standard -- asking if his commands are really good for humans, if his actions seem to advance the common good, if his actions specifically are what leads us to better moral development -- honestly, I find myself disappointed. The good stuff all seems to be derivable from human reason, and the bad stuff is hard to explain away.
Christianity is morally very impressive when people focus on stuff that even secular people agree with -- love, kindness, nonaggression, concern for the poor. But it also has an ugly side, that appears when fundamentalists take the Old Testament or certain bits of the New as a moral guide. It's like having a big smorgasbord of good and bad stuff, and if you eat from the wrong side of the buffet, you wind up with a moral disaster. Are you really going to get something much better, morally, from this smorgasbord than you get from unaided human reason?
If God is really guiding us, the answer should be obvious. Instead it is much less obvious than I would expect.