Reason ignores men of flesh and blood in favor of concepts like truth, goodness and beauty. That is why Kierkegaard was so hostile to Hegel's philosophy of pure spirit. Hegelian idealism drained all the blood and passion out of men. Reason is the original sin because it dissects life by logical analysis, turning passionate individuals into logical categories. Fourthly, reason destroys men's ethical strivings.
Rationalism transforins morality into eternal laws.
Man no longer has a choice. He is coerced into obeying. To conclude, existentialists identify the Fall of man with his reliance upon reason because reason is totally anti-religious. Reason stresses man's thoughts whereas religion stresses his feelings. Reason ignores the individual in favor of the universal, but faith is concerned with the unique value of each man.