Wielding Hegel's dialectic method, Kierkegaard set out both to refute Hegel's conclusions, and to simultaneously defend and to problematize the question of faith and the meaning of life.
Apart from Socrates and Nietzsche (the former who invented the concept, the latter who gave it its most paradoxical twist), no other philosopher has epitomized the concept of irony quite so powerfully and masterfully as Kierkegaard.
His thought is sometimes difficult to pin down, partly due to the fact he wrote under a number of symbolic pseudonyms who expressed differing points of view (Johannes de Silentio, Hilarius Bookbinder, Constantin Constantius, Johannes Climacus: total porn nom de plum, by the way), and partly because the dialectic process he embraced is inherently dynamic and ever-changing. Just to give you a small taste of the incredible kind of balancing act he sought to perform, Kierkegaard argued against an increasingly secular public that faith is an immediacy higher than that afforded by reflection and the intelligibility of universal ethical categories. And arguing against maudlin conceptions of faith (all-too-common today), he contended that faith ought to be experienced in the shudder of existential fear and trembling. To strike this balance of a higher existence to be experienced in anguish, Kierkegaard explores the question of whether Abraham was justified in being willing to sacrifice his son Isaac in such a way that, whatever perspective you come from, you can't help both to admire Abraham and to be horrified by him.
But instead of letting me yap endlessly about this master of philosophy, here's a nice introduction to Kierkegaard and his thought, narrated by the influential non-realist (or anti-representationalist) philosopher of religion, Don Cupitt:
Happy birthday, Soren!