My thinking about faith -- in every sense -- is informed by Fear and Trembling; its conceptual progression from stoicism to faith is as applicable to art and to daily life as it is to religion proper. The three pages of the Epilogue are perhaps the most perfect reflection on the nature of "progress" I know.
Complex, rich, maddening, and beautiful, Fear and Trembling is probably Kierkegaard's best-known work. I thought it was idiotic the first time I read it, but that was at least five or six readings ago. I've already written about this book elsewhere, but one could spend a lifetime unpacking its subtleties.