Some folks (myself included, when I was Protestant) suppose that natural law provides the Christian with no common ground on which he may stand when arguing for the truth with unbelievers. I can’t speak for everyone who takes this position, but my opinion at the time was that the non-Christian is so corrupted by sin as to willfully reason wrongly—so that natural law was functionally worthless. I no longer hold this view. I think that it’s pretty clearly wrong.
For starters, logic is not one thing for Christians and another for atheists (for example). One exercises the laws of reason properly or he doesn’t, but if he doesn’t, it’s not because the laws of reason vary. Logical demonstrations can only be wrong if one or more premises are false, or if the conclusion does not follow from them. And this brings us to the second point. Often when people reach different conclusions, it is not because they reason differently but because at least one of them starts with one or more false premises. This being the case, it is at least theoretically possible for men to reason together and to overcome their differences. And because truth cannot contradict truth—because there is no discord between the truths revealed by God and the truths discoverable by reason—we know that right reason will correspond to the truth revealed by God. Consequently far from being an exercise in futility, the natural law ensures that we do have common ground with others and that by reasoning together it is possible to arrive at truth.
A third point is that far from being constitutionally opposed to it, people are naturally oriented towards pursuing the good. They get things wrong not by pursuing what they think is evil, but by pursuing what they erroneously suppose to be good. Consequently we can reason with them and thereby show them that the good they seek is not the real Good (which is God), and encourage them to seek Him instead.
Aquinas, Etc. 
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