Natural law is an instance or instantiation of eternal law. Because natural law is that which human beings determine according to their own nature (as rational beings), disobeying reason is disobeying natural law and eternal law.
Thus eternal law is logically prior to reception of either "natural law" (that determined by reason) or "divine law" (that found in the Old and New Testaments). In other words, God's will extends to both reason and revelation.
Sin is abrogating either one's own reason, on the one hand, or revelation on the other, and is synonymous with "evil". Thomas, like all Scholastics, generally argued that the findings of reason and data of revelation cannot conflict, so both are a guide to God's will for human beings.
To propose that reason must coincide with faith is a radical position in the modern political climate. As the collective human knowledge base expands through new discovery, then it follows that articles of faith that originally filled a void in human knowledge might have to change. But along the line, some Christians picked up the notion that articles of faith handed down can never change even if they fly in the face of facts and reason. Better to cling to ignorance than seek the truth. Don't look for that message in the New Testament.
For example, many conservatives support the theory of young earth creationism, an anti-science that defends their religious belief that the creation of earth pretty much matches the literal account in the Book of Genesis, therefore the earth is only about 6,000 - 10,000 years old. To them the underlying truth is that if it was written in the Bible it must be true.
Informed and educated people know there is overwhelming scientific evidence and analysis proving the earth is actually billions of years old, not just thousands. Any person with access to information and a science education would have to be willfully ignorant and abrogate reason in order to accept young earth creationism as a viable theory. They promote anti-science and work to prevent public schools from teaching real science to the students lest it offend their religion. Apparently God's existence relies on their mission.
If you think this is only a small fringe of the GOP, peruse the website Conservapedia where true conservatives (not the RINO variety) go to find out what to think about any given topic in order to pass the conservative litmus test. Every single Conservative talking point is listed there. And for your sheer entertainment here is how they break down the science behind the age of the earth for the Conservative faithful.
The Age of the Earth has been a matter of interest to humans for millennia. The subject is still debated today, particularly between young-Earth scientists, who explain that the Earth is only approximately 6,000-10,000 years old, and Old Earth creationists who believe that Earth is approximately 4.5 billion years old. The scientific evidence points to a young age of the earth and the universe, and the biblical creation organization
The fatal flaw in Old Earth arguments is their overreliance on the assumption that the rate of radioactive decay remains constant over thousands of years. In fact, a large number of physical processes, such as neutron capture and fluctuations in solar radiation, can affect the rate of radioactive decay of elements in the Earth's crust and render radioactive dating measurements unreliable with errors up to 5%, depending upon the specific methods used.Abrogating reason. Behold true Conservatism in 2012. Keep in mind GOP candidates have to suck up to these people and promise to push their creationist agenda in the public schools to gain their support.
Many centuries ago, before the microscope or printing press were invented, Thomas Aquinas had a more sophisticated perspective about the interaction of faith and reason than the true conservatives do today.