Here's a fun video dealing with Alvin Plantinga's treatment of rational Christian belief.
In the 1600's Sir Isaac Newton posited what we now call Newtonian Mechanics or Newtonian Physics. Newton was able to explain both the movement of things on earth as well as those in the vacuum of space. Many of Newton's conclusions "worked." That is they had considerable explanatory force and scope. Then along came Albert Einstein and his theory of special relativity. Einstein's theory was able to account for the movement of large objects [e.g., footballs and planets] as well as the effects that electricity and magnetism have on these objects. It is not that Newton's theory was "bad." It is simply that Einstein's theory could explain more and was therefore a better theory. In short Einstein was able to account for Newton's observations and then account for other phenomena beyond those observations.
Much of life is this way. We once changed the oil in our car after this or that fashion and now we take it to the Take 5 oil shop. We once washed the dishes in this or that fashion and now we use the dishwasher. We once thought that this or that political party's platform was the most comprehensive and internally consistent. Now we don't. We once understood free will to be of this sort and of these implications and now we think differently. Presumably, in all these cases there is a shift in systems because the old one held less explanatory scope and force compared to the practice or position we currently hold. Consider that these kinds of shifts and explanatory considerations may also happen in a given person's moral landscape.
At one point you may have thought that owning guns was a good thing and now you are of a different persuasion. Perhaps you once thought homosexuality to be base or wicked and now you see such behavior a mere personal preference or biological prewiring. Perhaps you once thought that God kept a moral record of the things you thought, the words you said, the things you did, and the places you went, but now you think there is no God at all to care about such things. I submit to you that just as we adhere to scientific systems and day-to-day systems we also adhere to moral systems. Furthermore, we can both agree that these systems "work" to varying degree and when we find a better system, one more to our liking even, we tend to change from our current system and move to the new and better one.
Lord willing, in the weeks to follow I would like to discuss with you the idea that you and I both have a system of morals and, generally speaking, we believe that our respective systems of moral beliefs are able to account for our moral stances on this or that event or behavior. So how robust do you think your system of moral beliefs is?