St. Thomas Aquinas's seminal work the Summa Theologica was an attempt to catalog all the theological arguments ever conceived.
Much of the reasoning is rooted in the Natural Law and it's what Catholics believe today. It's the reason we've got such strong opinions on abortion, birth control, embryonic stem cell research, homosexual "marriage" and more even though private interpretation of the Bible could potentially lead one to believe otherwise.
What I wonder, is how much of a common ground is the Natural Law (specifically as it relates to the Summa) and how did the reformers (Luther, Calvin, Wesley, etc.) decide whether or not to accept it?