G. K. Chesterton called original sin the only part of Christian theology which can really be proved.
Certain new theologians dispute original sin, which is the only part of Christian theology which can really be proved. Some followers… in their almost too fastidious spirituality, admit divine sinlessness, which they cannon see even in their dreams. But they essentially deny human sin, which they can see in the street. The strongest saints and the strongest skeptics alike took positive evil as the starting-point of their argument. If it can be true (as it certainly is) that a man can feel exquisite happiness in skinning a cat, then the religious philosopher can draw only draw one of two deductions. He must either deny the existence of God, as all atheists do; or he must deny the present union between God and man, as all Christians do. The new theologians seem to think it a highly rationalistic solution to deny the cat, (pg. 28).