Human Nature and Morality
In view of the last post's little sketch of the way in which it is important to keep human nature in view when setting purposes for ourselves, we can see that M. Stanton Evans's view of the role of God in a deriving morality from human nature is not a necessary role. Indeed, it may not even be helpful in view of what I said about nihilism in the aside.
Still worried? Keep your oughts close to your ises. The reason you ought to set purposes in harmony with observations, wisdom and lore about your nature is that you will be unhappy and unfulfilled if you do not. Also, in many cases you will go well enough astray to end up with an immoral way of life which disposes you to do wrong by others.
There is a link between what is right and human nature, though it is not a direct logical entailment of the former by the latter or any simple identity of what is right and what human nature bids us do. It is rather a loose tethering between the two, though a tethering just the same. There are cases in which we ought to disobey what nature bids. There is no need for a God to fix morality with the cement of human nature. Right and wrong would be what they are regardless of his choice.
If you're still worried, then go ahead and have your God. It's a beautiful thing to do and one more profound than words can say. It may make sense to believe in Him even while recognizing that his existence isn't necessary to morality.
Anyway, all that is not the point. The point is what human nature and morality are and what their relation is. Here is another bit of the linkage. Human nature equips us with benevolent or altruistic inclinations: desires that others fare well. Human nature also, of course, equips one with strong regard for one's own welfare. Observations, wisdom and lore about your nature allows you to understand the coherent meshing amongst benevolent and self-interested desires that may be attained. The manifold and complicated ways in which they coalesce in a set of values are a culture which reflects human nature. There will be more than one reasonable culture, where "reasonable" refers to internal coherence and degree of consistency with human nature and various other relevant facts about the way the world works. The attainment of a character in harmony with this coherent meshing of desires is virtue.