The Pope addresses the importance of believers and unbelievers reaching an understanding about the truth of human nature. Such an understanding will lead to a concept of natural law, which should be a guide for a government. Natural law is not based on a particular religious revelation, but on reason. Natural law protects against moral relativism at the personal level, and tyranny at the level of government.
St. Thomas Aquinas did a lot of work on natural law because although he followed Christian revelation, he wanted to determine what could be known by human reason. Aquinas did not see a conflict between faith and reason, but instead saw them as two avenues to reach the truth, which ultimately comes from God. There are some truths which cannot be known through reason alone, but what is known through reason is compatible with Christian faith. Faith allows a person to move beyond the limits of human reason into the mysteries of God, not destroying human nature, but building upon and perfecting it.