The doctrine of original sin shows the need of the remedy of our Saviour's Incarnation and Passion.

The doctrine of original sin and the consequent corruption of human nature it entailed is fundamental for understanding the Mystery of the Incarnation of the Son of God and our need of that remedy. It should be noted that Divine Providence has made use of two remedies for man’s sanctification—one is contained in the law of nature, the other in Holy Scripture; for by the former law the knowledge of good and evil is imprinted in men’s hearts with the decree that they should practice the first and hate the other. He also gave them a natural love and reverence for God resembling that of children for their father. Besides this natural interior inclination there is one that is exterior, for the sun, the moon, the stars, the beauty of the movements of the heavens, the changes of the seasons, the development of things, and finally all creatures say:- “ God made me.” The animals, with the perfection of their bodies and the instincts God bestowed upon them for their preservation, especially incite us to this love and reverence (cf. St. Augustine, Confessions X).

The result of the natural law was that although some men were just and holy, the universal punishment of the flood shows how small was their number compared with that of the wicked.