…To become a Christian requires a 'leap of faith' without the safety net of reason or evidence.
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.
That is not because Kierkegaard was guilty of an anarchic irrationalism or relativistic subjectivism. It is only because he was so rigorous with his application of reason that he was able to push it to its limits. He went beyond reason only when reason could go no further, leaving logic behind only when logic refused to go on.