Simon,
Love the question. I think your question was answered strongly by Kierkegaard in Fear and Trembling. His answer is the exact opposite of Earnest and is intellectually much more satisfying.
The ethical expression for what Abraham did is that he was willing to murder Isaac; the religious expression is that he was willing to sacrifice Isaac.
The ethical is the universal....
Faith... is this paradox, that interiority is higher than exteriority.
Then faith's paradox is this, that the single individual is higher than the universal, that the single individual determines his relationship to the universal through his relationship to the absolute, not his relation to the absolute through his relation to the universal. The paradox can also be put by saying that there is an absolute duty to God; ...if this duty is absolute the ethical is reduced to the relative....e.g. love of God can cause the knight of faith to give his love of his neighbour the opposite expression to that which is his duty ethically speaking.
....In the story of Abraham we find just such a paradox. Ethically speaking his relationship to Issac is this, that the father is to love the son. This ethical relationship is reduced to the relative as against the absolute relation to God... The absolute duty can then lead to what ethics would forbid... this is shown by Abraham... the ethical expression for what he does is this: he hates Isaac.
This is the "fear and trembling" which Kierkegaard felt his contemporaries lacked. Most current believers who would only believe in a God who conforms to ethics fall under this category. They may have something they call "faith", but it is not the faith of Abraham.
I'm an athiest, though not a particularily strong one. However, I do have immense respect for the likes of Kierkegaard who understand and embrace the logical conclusions of faith. In my experience, few religous people truly grasp this and prefer to see faith as rational and as an expression of Universal Ethics rather than at war with it.
If God says kill your son, you do it. If God says rape your son, you do it. And here is the monsterous elegance of faith. What a powerful and insane concept.
So let us either forget all about Abraham or learn how to be horrified at the monstrous paradox which is the significance of his life...
But, if one wants to market a cut-price version of Abraham and then still admonish people not to do what Abraham did, that that's just laughable.