So natural law is the ability of human persons to use the gifts of free will and reason, to consider the good found in Creation, especially the good found in human persons, in order to understand moral truths.
But original sin, our personal sins, and the sins of society, all make it more difficult to achieve a full and proper understanding of morality. The free will is weakened and reason is clouded by sin.
Therefore, God has given us Tradition and Scripture and the Magisterium by which we can understand moral truth more easily and more fully than by fallen human reason alone. And this in turn allows us to adhere to moral truth more firmly, because we are more certain that we have found truth and not error.
Roman Catholic theologian