This is however, precisely the point. The one who previously has been among the sinners however, and has subsequently repented, can now truly despise all forms of sin. From here on when he chooses the righteous path, he can legitimately testify that the good is truly good, worthy of being chosen as a way of life. Rabbi Joseph B. Soleveitchik discusses how realization of G-d can be enhanced by some very serious contrasts. Sin reveals to man the beauty of good.
Crime reveals the glory of the ethical.
Mental disintegration reveals the enchantment of spiritual perfection. Values lost are more fascinating than ideals which are fully realized. Terach, we are reminded in the Haggadah, was an idol worshipper. But of all his sons, one son, Abraham, was able to discern the contrast phenomenon that Rabbi Soveveitchik speaks of. Isaac in his day too discerned this, and passed on the blessings to Jacob.