It's that last clause that is the most problematic: when we come to the end of reason, logic, morality, and evidence, we are not faced with a chasm, which we must then take a leap of faith across. That would be bad enough--if you wanted to put me off the idea of something forever, you would have merely to describe it to me in much those same terms. But those terms are incorrect, and the real difficulty is infinitely worse: at reason's end, there is not a chasm, which must be leaped, but a multitude of chasms, which any would-be leaper must choose between.Kierkegaard’s greatest illustration of this is his retelling of the story of Abraham and Isaac in Fear and Trembling (1843). Abraham is often held up as a paradigm of faith because he trusted God so much he was prepared to sacrifice his only son on his command. Kierkegaard makes us realise that Abraham acted on faith not because he obeyed a difficult order but because lifting the knife over his son defied all morality and reason. No reasonable man would have done what Abraham did. If this was a test, then surely the way to pass was to show God that you would not commit murder on command, even if that risked inviting divine wrath. If you heard God’s voice commanding you to kill, surely it would be more rational to conclude you were insane or tricked by demons than it would to follow the order. So when Abraham took his leap of faith, he took leave of reason and morality.How insipid the modern version of faith appears in comparison. Religious apologists today might mumble about the power of faith and the limits of reason, yet they are the first to protest when it is suggested that faith and reason might be in tension. Far from seeing religious faith as a special, bold kind of trust, religious apologists are now more likely to see atheism as requiring as much faith as religion. Kierkegaard saw clearly that that faith is not a kind of epistemic Polyfilla that closes the small cracks left by reason, but a mad leap across a chasm devoid of all reason.
A leap of faith, you say, beyond reason and evidence? Very well, I shall die heroic in battle and be escorted across the rainbow bridge by a beautiful blonde war-maiden, there to feast in the company of Wotan and his heroes until the clarion call of the Gjallarhorn summons us forth to die in fire and glory at Ragnarok.
Oh ... you meant a leap of faith beyond reason and evidence towards your god as opposed to all the others that ever were. Ah. Well then. But why, exactly, should I leap across this chasm as opposed to that? Be sure not to use reason, logic, or evidence in your answer, because once you've allowed those into the equation, the only possible conclusion is not to leap.
This is a very grave problem, and one with very real consequences: suppose Abraham was right to defy all morality and reason and ready a knife to plunge into Isaac, to be stopped only by an angel's timely intervention. On what grounds, then, were the 9/11 hijackers wrong to defy all morality and reason and ready a plane to plunge into a building? If there ever was a mad leap over a chasm devoid of all reason, then surely that was it! And yet we rightly abhor them--as we should abhor Abraham as well. The willingness to ignore morality and reason and kill for the voices in your head--that you distinguish from all other such voices, and furthermore trust, on unspecified grounds--is the mark of the psychopath, not the saint.
In the end, the leap of faith isn't so much an argument for faith as it is the admission that there aren't any good arguments for faith, and that we should do it anyway. But, having dispensed with arguments, we find that without them we are powerless to determine what exactly the 'it' should be.