“For whenever the Gentiles, who do not have the law, do by nature the things of the law, these, although they* do not have the law, are a law to themselves, 15 who show the work of the law written on their hearts, their conscience bearing witness and their thoughts one after another accusing or even defending them 16 on the day when God judges the secret things of people, according to my gospel, through Christ Jesus.” 
You are probably familiar with the moral argument for the existence of God, but just in case you are not, it goes like this:
- If God does not exist, objective moral values and duties do not exist.
- Objective moral values and duties do exist.
- Therefore, God exists.
If you follow my blog, you know that I am unsure of this argument. The question for me is premise one.
Let me be clear from the beginning: I am not saying that this argument fails. I am saying that I have questions that I don’t see anyone else asking. Maybe they are, but I haven’t found them.
As a Christian who believes God’s word to be the final authority, I find in the above verses the idea that Gentiles (in this passage, a reference to non-believers) can know what is right or wrong apart from God’s commands.
Now, this is usually skirted by saying, “Yes, that is because human beings are created in the image of God.”
Very well, my question is this: How can you know something without having access to it?
These Gentiles did not have God’s law, that is God’s special revelation. And divine command theorists seem to be saying that moral values are grounded in God’s commands which necessarily follow upon God’s good nature.
Well and dandy, but how could someone then know these moral values a part from any divine command? It seems to me that if it is possible to know moral values apart from any divine command, then moral values must not be grounded in divine commands.
I’m quite sure divine command theorists have a response and I look forward to them. I’m merely asking questions, not trying to attack.
Secondly, I would point out that the above verses actually use the word “by nature” to describe how these Gentiles know right from wrong apart from divine commands.
This sounds oddly familiar. Aristotle, and Thomas Aquinas after him, grounded moral values in the objectivity of human nature. Of course, God is responsible for human nature, and that is how Aquinas argued for God’s existence, but that is a cosmological argument, not a moral one.
Moral actions are human actions. Every action could be judged morally. An action is morally good if it perfects human nature. It is morally bad, or evil, if it depletes human nature.
This is known by all because it is grounded in all — that is human nature. If this contention is right, it seems that premise one is false.
Even if God did not exist, as long as human nature still did, then morality would still be objective.
The problem with objective morality on atheism isn’t really the lack of God. The problem is evolution and determinism, which proponents of the moral argument always bring up. But this is separate from a mere lack of God.
Determinism is a defeater of any objective moral duty. There’s no way a person can be morally culpable without libertarian freewill. If I push you into someone and they fall and break their arm, are you responsible? Of course not, I made you do it.
Evolution would seem to defeat objective morality as well. For then, our human nature and beliefs would be the result of a blind process that is aiming, not at truth, but at survival. In this way, even if morality was objective, we could never justify our belief in it.
In summary, I do not believe the moral argument necessarily fails. However, I do not understand morality the same way proponents of the argument do and therefore shy away from it.
Perhaps a better way of arguing would be to argue from free will, or to argue to human nature and then show how God is the explanation of human nature using a cosmological argument.
Again, these are just some thoughts. I hold them all tentatively. Please feel free to respond and correct me.
 Harris, W. H., III, Ritzema, E., Brannan, R., Mangum, D., Dunham, J., Reimer, J. A., & Wierenga, M. (Eds.). (2012). The Lexham English Bible (Ro 2:14–16). Bellingham, WA: Lexham Press.