The idea that the Bible should be interpreted as a metaphor is a good one — because it melts the superstition away.
The metaphor is a powerful tool: it allows a new idea to be expressed in old and familiar terms as an aid to comprehension, and it also allows the lumbering in of a freight of emotion that can add resonance to the ideas. I use them, everyone uses them, I believe the authors of the Bible used them heavily, and they're great; without them, you're left with simple literal descriptions, turning a literary effort into a textbook.
To some people, though, metaphors are anathema. The raison d'etre of Answers in Genesis is an obstinate denial of the art of the metaphor, insisting that every word of the Bible must be a literal recitation of the dry facts, straight from the mouth of God, and that the Bible really is a kind of supernatural textbook with an absolutist interpretation of history, science, human behavior, and morality. Apparently, God never needed to couch his explanations in any terms other than the straight, boring facts, because his prophets all understood all the context and all the implications.
But AiG is right about one thing: opening the Bible up to metaphorical interpretation rips the heart right out of Christianity, and makes central dogmas of the faith untenable and painfully ridiculous. And not just Ken Ham's hick literalism gets eroded away, but even the more sophisticated apologetics of more scholarly theologians becomes silly and irrelevant.
So let's take a look at the central message of Christianity, first as a literal description of a hallowed reality, and then as a metaphor for something deeper and more human. Let's see what survives.
First, the key literal events of the Bible for a Christian.
The most important story in the book of Genesis is the Fall. God gives Adam and Eve a paradise, and asks of them one demand: they are not to eat the fruit of one particular tree, which is rather arbitrary, but this is a magic tree that will give them knowledge and understanding. Tempted by an evil spirit disguised as a talking snake, though, Eve and then Adam eat the fruit, and become self-aware and modest. God finds out (because he knows everything), gets very angry, and casts them out of paradise for their disobedience, cursing them with afflictions that will also be passed on to all of their descendants, that is, the whole of the human race.
The most important story in the New Testament is that God finally forgives humanity, and he cobbles up a strange mechanism to give everyone a second chance. The roundabout way we're to be forgiven is that God turns himself into a human being, visits the earth and taunts the secular and religious authorities until they get fed up and torture and kill him in horrible ways, and then the god comes back to life. Now we're forgiven for our many times great grandmother's act of disobedience if we simply really, really believe that Jesus was a resurrected god. Again, it's rather arbitrary: God is flinging up very silly demands, that we don't eat this fruit or that we do believe in this miracle, and we get to be in his good graces if we grovel enough and do as he says.
Note, though, that God doesn't actually retract any of his heritable curses if we do obey his second demand, which is a real shame. It would be very persuasive to the rest of us if believing Christians were suddenly exempted from that 'toil to the end of your days' requirement, and Christian women didn't feel any pain in childbirth anymore. But no, Eve's curse still applies, you're only really forgiven once you're dead.
It's an absurd and unbelievable story.
I don't believe in inherited curses. If I were caught jaywalking, punish me…it would be unjust to then impose a monthly fine on all of my descendants because great-great-grandpa took a shortcut one day. And this is supposed to be a just and loving god, right?
I don't believe in blood sacrifice. My descendants should not be relieved of their fine because some fine fellow volunteered to get his flesh shredded and to die a slow death — that solves nothing, excuses nothing, and the kids shouldn't be penalized for an ancestor's actions anyway. What kind of just and loving god feeds on death and pain?
I don't believe in forgiveness by proxy. If I do wrong, it is my responsibility. No one else can take on my errors; no just and loving god would accept such a pointless sacrifice.
I don't believe that mindless obedience is a virtue. These are stories where a god imposes skewwhiff random rules on people, and rewards them if they obey, and punishes them severely if they don't. This is not the work of a just and loving god, but a psychopathic tyrant.
It's a story that should be rejected as a literal tale. It's a metaphor for something, translated into language that a patriarchal culture with some primitive and peculiar notions of crime and punishment could understand. A lot of Christians don't grasp that (see Ken Ham and nearly all young earth creationists for examples), but I will give credit to many other more sensible Christians — they understand that it has to have some deeper meaning, that you shouldn't try to interpret what should be a universal message for all of humanity through the lens of backwards iron age goatherders, and that maybe the wisdom lies in the metaphor. Good for them. That's a step towards understanding.
Of course, I'm a right cruel bastard, so when a liberal Christian tells me that Genesis and the sacrifice of Christ are metaphors, I just ask "Metaphors for what?", and then they usually stand there gape-jawed like a fish and flounder trying to figure out what I'm asking. Calling something a metaphor is not a get-out-of-jail free card. It means there's a deeper meaning to extract.
There are certainly more scholarly Christians who can explain how to interpret a metaphor. Some do answer. Others clam up, not because they're stupid, but because they know if they go down that road openly it can quickly make the religion superfluous…and seriously, if you want to piss of the majority of Christians, try to get them to explain the crucifixion of Jesus as just a metaphor and not as a real historical event with direct metaphysical consequences. It's not pretty.
So let's consider the key tenets of Christianity as metaphors. These are interpretations that liberal theologians make, but surprisingly, they're also perfectly copacetic with atheist and humanist ideals.
The book of Genesis is telling us that human beings are flawed, that we're all burdened with impulses and desires that are not necessarily good for our society: greed and selfishness, for instance, or violence and deceitfulness. (And also, to a patriarchal society, disobedience — conscientious objections don't seem to have much support in such cultures). The whole of Genesis, not just the creation stories, is about the natural wickedness of human beings, and how we have to be constantly chastised.
You won't find a single rational person who disagrees with that. We are not angels by nature. We biologists would go even further and say that by nature, we're fractious, squabbling apes. Read that as the lesson of Genesis, and you'll find even us rabid militant atheists in full agreement that it is right. The mythological details are nonsense, of course, but they're just there to make it an interesting and persuasive story.
What about the metaphorical interpretation of the Jesus myth?
That's a hero story, a narrative about someone we should emulate, whose greatest virtues are self-sacrifice for the common good. We're wicked deep down as Genesis tells us, but we can also aspire to believe in humanity and give our lives over to charity and justice.
Again, this interpretation is not going to conflict with most godless values (well, unless you're an Ayn Randian, but those are psychopathological aberrations). We're combative apes, but our species also succeeds through cooperation; we have a 'higher' nature to which the best of us can appeal, which has and will help us succeed. Maybe believing in something greater, like sacrifice and hope, can help us be better people.
This is what should make the believers uncomfortable, though. None of these interpretations require a literal god or Jesus, Christianity, the Bible, the church, the priests, the rituals, the offering plate. This is something Richard Freakin' Dawkins can say. The faith, the superstition, the sectarian tribalism, the whole edifice of supernatural religious thought is dispensable. Making the message of their religion universal and human dissolves the superficial linchpins of their faith.
You don't need to believe in Jesus in order to be good? The Hebrews were not a supernaturally chosen people, but just one among multitudes using stories to make sense of their world? The Bible might have a broader meaning, might be representative of human aspirations in the same way as millions of other works of literature, and requires more thought to understand than memorizing verses?
Say it isn't so. It's so much easier to cling to the trappings and follow the rituals than to actually consider meaning. Unfortunately, that's what addressing the metaphorical meaning of their religion demands; the whole concept of a metaphor requires awareness of what parts of the story are the stage setting, and what parts are important.
Interpreting the Bible through metaphor is a kind of blasphemy: it's like spraying it with acid. Most of it burns away, and all that's left is a story about people trying to explain the universe as best they can, with no gods, no magic, no angels, no devils. It's also a useful test of the people of the book. Are they concerned about life and culture and their fellow human beings, or are they more interested in worshipping wood pulp and ink?