A good comparison would be the moral law. If each person relied on his own opinion to determine what was right or wrong, we would have nothing more than moral relativism, and each person could rightly assert his own set of standards. However, since God has clearly defined moral absolutes for us (in addition to those we can know by reason from the natural law), we can assess any given action and determine how morally good or bad it is. This would be impossible without moral absolutes.
