The whole point of the Bible, and the reason people look to it for guidance, is that it makes the case that there is a notion of a moral absolute, a right and wrong that is not derived from cultural norms, but from elsewhere. The elsewhere is the realm of the spirit, and the notion of God tells you that if you look to this realm with a calm mind and a good heart, you will receive guidance.
This insight is intuitively plausible for nearly all people, since we come with a built in moral compass that tells us generally that some behaviors are right and some are wrong. But this moral compass, the intuition, is sometimes faulty, in that it leads us to condemn the action of others as immoral when it conflicts with our own instincts about what we would do in the same circumstance. It is also easily manipulated by culture or habit, doing something terrible many times can numb you.
The position that there is a correct answer to moral questions, contingent only on circumstances not on culture, is the position of moral absolutism. This means that given identical circumstances, all people should come to agree on the right and wrong. Culture counts as circumstance, of course, so that in a Christian culture it might be ethically required for a person to send a Christmas present to his aunt, while in a Buddhist culture, it wouldn't be. That's not relativism. Moral relativism is saying that if you asked the Buddhist culture to think about what is ethically required of the same Christian person living in the same Christian culture, and describe these circumstances in detail, they might decide that it is not ethically required to send that present, and that their position would be just as good. This is what moral absolutism forbids.
Given the diversity of human culture, this is a strange idea. If a 19th century Comanche warrior captured you, his culture taught him that the proper course of action is to either torture you to death, if you are a man, or to sexually assault you otherwise. This type of thing was culturally embedded, so that a warrior would pride himself on his ability to be stoic in the face of pain, and would feel humiliated if he betrayed any sign of suffering. The torture would be a test of your stoic spirit, and after you failed the test, you deserve all the further suffering you endure. It is this sort of hard-to-eradicate socially sanctioned self-perpetuating barbarism that the Bible is designed to stop.
The idea of universal ethics is that this reinforcing social setup is wrong, not by my personal standards, or by your standards, or from one point of view, but even from the point of view of the Comanche, or of anyone else. It is just not right, period. In other words, you look at the Comanche's actions, not from the point of view of their culture, but from the point of view of their victims, which is also the point of view of God.
In cases like these, it is very easy to be an armchair philosopher, to say "but if you were brought up in a culture that taught you that it is ok to torture captured victims, that you would do it too." This is possibly true, but it is irrelevant. It ignores the essential fact known to anyone brought up in a bad situation: you do it, buy you still know that it is wrong.
Personally, as an Israeli, I remember my sense of unease in the intifada year of 1987, the nagging feeling that I am on the wrong side. I knew very quickly that I could not serve in the Israeli army, which meant that I was headed to prison in a few years. I worried about this for a while, but then my family moved to the U.S., and this mooted the issue. But from experience, it's not your culture that tells you when you are doing wrong. If you don't know, it's because you've chosen not to.
The story of the Bible is designed to convey the idea that in the long struggles of history, the winning path is also the one that you know is right, in terms of these hard-to-define universal ethics. The way it does so is by identifying the source of the universal ethics as the lawgiver God, who lies above all other lesser gods. It attributes the creation of the universe to this God, and gives God unlimited powers to strike down the wicked. These stories are obviously literally false, God doesn't do squat, but they get the message across: over time, those cultures which condone unethical practices will not survive against those cultures which are ethically aware. This message is more or less true. It is about as true as any universal proposition about a complex world can be.
Since I mentioned the Comanche, and since arguments like these were used in the U.S. to justify genocidal policies, I have to say that genocidal policies are never the proper course of action, even when faced with an intolerably barbaric practice. Missionary work together with law enforcement can eradicate the barbaric practices while allowing the language and traditions to organically evolve to incorporate old-world insights. One doesn't have to hide Christian philosophy from people, but one also doesn't have to eradicate a culture in order to spread a message of compassion. Of course, in a situation where one is potentially watching a person get tortured to death, there is no choice, really. Desmond Tutu demonstrated the proper Christian response to such a circumstance , and one prays one can muster the courage to live up to his example.
If you are positivistically minded, as I tend to be, you want an in-principle algorithm to find this universal ethics, to be sure you know what you're talking about. Given that God's ways are the winning ways, the precise algorithm is to wait a sufficiently long time, and ask somebody from the future what they would do. This works backwards: any random person on the street would make a pretty good moral teacher for the cave-people of 10,000 years ago, for 19th century Comanches, and probably even for the mid-twentieth century people of Europe.
So here is the basic idea in the Bible: there is an objective ethical standard, the source of this ethical standard can be reasonably and accurately personified as a super-smart individual, and that this ethical standard is revealed approximately and ever more accurately gradually through the victories in the struggles of history.
There are more ideosyncratic things in the Bible, like the rules for washing your hands, and sacrificing pigeons after your period, but I will ignore these. The point of the Jewish rules is to establish an ever-present sense of God's presence in day-to-day activities, so that you are never caught in a situation where you are not mindful of God. Similarly, in Christianity, you are taught to constantly be aware of your sinful nature, so that you think of Jesus every time you have some sexy thoughts. In Islam, you pray 5 times a day, and submit your soul to God, asking for mercy. All these practices are somewhat effective on the social level at eradicating barbarous practices. They worked to eradicate child-sacrifice and temple prostitution in ancient times, and slavery and racism in modern times.
To see that this actually does work in real life, you should consider how socially sanctioned crimes were carried out in atheist states. In Nazi Germany, Hitler began a euthanasia program for the mentally disabled, which was the pilot precurser to the holocaust. So many Catholics and Protestants were outraged by this program, despite government propaganda about its value, that the Nazi government was forced to shut it down. The SS learned the lesson--- genocidal policies would be carried out in complete secrecy from this point on, and the religious folks were kept unaware. Still, some Catholics made it their business to be aware, and formed the White Rose. Their awareness of God was their only defense against a criminal social order.
Likewise, the Catholic church in South America, along with Marxist organizations, were and are giving voice to dispossessed indigenous peoples. Protestant churches in the U.S. were central in the abolitionist movement. In every case where crimes are perpetrated, the church serves as a collective conscience, informing the state of universal ethics. Where it is suppressed, or inactive, barbarous practices flourish.
Mathematical models: God and Games
My scientific upbringing makes me feel uneasy about advocate such a vague-seeming notion as universal ethics, coming from a universal lawgiver which is not directly observable. I don't like to propose an idea without a mathematically precise argument. The idea of an ethics-giving personal God is a pretty implausible pill, even with a mathematical argument. I will try to argue that there are good reasons to believe this, without relying the pseudo-historical made-up narratives in the Bible, and without relying on personal revelation, compassion or empathy, or any other moral instinct, only on game theoretic considerations.
At the risk of alienating every single one of the readers, and inviting a thousand downvotes, I will describe a mathematical model which will show how this type of uber-person decision making can be seen to emerge from a bunch of people playing games. Further, it is plausible (although it cannot be strictly proved) that this decision making process is convergent, that there actually is a sense in which a universal ethical standard will emerge over time. This means that the religious sense will not be exclusively human--- it should be shared by intelligent alien life, or by artificial intelligences, if sufficiently developed. It is a meta-property of collective and individual decision making, which must be recognized in order for collectives to function correctly, to maximize their collective potential.
First, I will explain Nashian game-theoretic decision-making, and why it fails, for the simple case of symmetric games. This discussion parallels Douglas Hofstadter's discussion of superrationality in 1980s Scientific American, reprinted in "Metamagical Themas". The extension to non-symmetric games is standard religion (to distinguish from non-superrational systems that call themselves religions too, like Levay's Satanism).
Toy Model: Symmetric Super-rationality
Consider a prisoner's dilemma with symmetric payoffs and very little temptation. This means, you and your opponent are both placed in a room, both of you have a button on the wall, and if you push the button, your opponent will be killed and you will get a dollar. If you don't care whether your opponent lives or dies, and neither of you is suicidal, what is the correct course of action to maximize your probability of survival (and perhaps get a dollar)?
The game theoretic answer is to push the button. Push that button quickly. Just in case the other person decides to do the same thing. This solution defines game-theoretic rationality, the rationality of economic behavior. I will call this "Nash rationality" after John Nash.
In order to not be so morbid, and so as not to trigger killing aversion instincts, and so on, this game is usually not described as fatal--- you can suppose that if neither of you presses the button, you both get $100, but if your opponent presses you get $0, your opponent gets $101, and vice-versa, and in the case that both of you press the button, you both get $5.
Two ostensibly rational economists in this situation will walk out with $5. The point of religion is to make sure that most of us will behave less stupidly than those economists, so that we can enjoy the $100 prize. There is nothing blocking the two opponents from that prize except for their own button-pressing constructed rationalization.
To avoid this rationalization, Douglas Hofstadter pointed out that one should take into account the following fact: whatever course of action I take, the other person will take the same action. Hofstadter suggested that one should try to maximize the payoff assuming that my action will be perfectly correlated with the other person. If you do this, there are only two options, really, both of us cooperate, or both defect.
This idea defines a different type of rationality, also self-consistent, called superrationality. These two self-consistent modes of behavior are both observed in people playing the prisoners dilemma, and it is not possible to logically rule out one or the other.
But it is possible to advocate that one makes sense and the other doesn't. I believe that the superrational course of action is the obviously correct one, when one is playing against a superrational player.
But in the real world, how would you know if the other player is superrational? You need some sort of signal that the person is superrational. To do this, it helps if you have a secret handshake, a dress code, or a manner of speaking. It also helps if you identify each other publically in meeting places. This is the major purpose of places of worship, and the major purpose of religious identification. If you are playing against a religious player, you are guaranteed some form of superrationality.
I will defer to the Wikipedia article for more detail (I'm not copping out--- it was mostly written by me).
Nonsymmetric super-rationality is Monotheistic Religion
Superrationality, as Hofstadter discusses it, is restricted to symmetric games. You can only know that the strategy of the other superrational players is the same as yours in the case that they are solving the same problem as you. In the real world, games are never symmetric, and they can rarely be approximated as symmetric, so this is an artificial restriction.
But one can extend the notion of super-rationality by assuming there exists a monotheistic strategy--- a unique strategy for all games, which is to be followed by all players who consider themselves superrational. In order to be a real monotheistic strategy, it should reduce to the superrational strategy in the case of symmetric games.
For non-symmetric games, it will tell you how to play so as to maximize.... what exactly? The issue is that when you have a universal strategy, it might not maximize your personal utility, or the "sum" (whatever that means) of utility between all the players, or any such thing.
The only quantity which is sensible to talk about is the utility of a decision making entity, since utility is well defined only when you have an entity which can make choices between probabilistic options (this is the Von-Neumann Morgenstern utility theorem). The choices which are made by the superrational strategy are therefore those that maximize the utility of an uber-entity, a collective entity, partially formed by all the players.
I will call this entity a "god" (lower-case g). The superrational strategy is to maximize the utility of the god formed by the players in the circumstance. Since the strategy is universal, contingent only on the circumstances of the game, the gods themselves, in playing games against each other, all make choices which are consistent with the utility function of a super-duper agent, which lies on top of all the gods. This entity I will identify with the monotheistic conception of God.
A religion will be considered correct when the decisions it dictates for partisan games is consistent with the utility function of God. The convergence of ethics is then the statement that all societies reveal the decisions of God for all partisan games.
I have tried my best to be precise about what I mean, and I hope that the notion of absolute ethics can then be seen to be reasonable. In the next section, I will argue that the old-testament of the Bible is a rough approximation to this universal ethics, and the teachings of Christ somewhat more so, although these teachings are by now mostly incorporated into all the major existing religions.
The Bible as a source of Superrational Ethics
The Bible explicitly asks you to construct a notion of an external and unmodifiable entity, namely that of God, and to consider God's will in your day-to-day decisions. The Bible further makes claims about how God reveals His will, through the actions of history, through personal meditation and prayer, and through congregative religious practice.
I will argue that this biblical position is a statement that the God that one prays to is the entity whose utility coincides with the utility function of the perfect superrational strategy for non-symmetric games. That these are more or less the same notions.
The obvious problem with this identification is that the strategy God didn't create the universe, doesn't perform miracles, cannot respond when you pray, and is in general oblivious to the mechanical workings of the universe. The God of Christianity is a living God, which is everpresent and meddlesome. These seem like different concepts.
But the point is that each of the individual players in the game have an image of God, an approximation to the universal ethics, as a template for their mental strategy. This is a collectively useful thing, because a society which contains this template can function to expand its influence collectively, while a society which has a suboptimal template will fare worse. So the winners in the struggles of history will be those societies that have the most optimal approximation to the superrational strategy implanted in all their players.
In order to do this, the guide to the superrational strategy must be ubiquitous, simple, illustrated with homey parables and examples, persuasive, and generally spiritually moving, so that all people will encounter it, and pay attention when they do. This is the Bible. The Bible makes us aware that we have responsibilities to God, and makes us aware of God's presence. It isn't the only book which does this, and God knows it's not perfect or complete, but it's a start.