Let me say at the beginning - there is much in Natural Law that is true -- there are points that some people claim as Natural Law that I would quibble with, there are applications I would object to - but there is a Natural Law.
However, I fear that it's current upswing in popularity is coming from a Theology of Glory point of view (a point of view I think it can be attached to quite easily... pre-reformation Catholicism was the domain of the theology of Glory).
Now, why do I say this? Because Natural Law is being viewed as a means of changing the world, of improving this world, of making this place a better place, as though this world isn't being prepared for destruction and renewal. Natural Law is being viewed as the last, best hope we have for keeping society from spiraling off into chaos, for making people moral.
Here's the problem.
Natural Law is God's Law.
What is the first commandment? Thou shalt have no other gods before Me.
What does this mean? We should fear, love, and trust in God above all things.
If one is an unbeliever, who by definition neither fears God nor trusts Him, why in the world would you give a (insert euphemism of your choice) about God's Law?
Seriously. Why? Will someone who denies that we were designed by God care about our arguments that stem from God's design? Will someone who thinks that we choose to be whatever we wish to be care about the "laws" of nature that he, as a fallen creature, delights in ignoring anyway?
But, but, but if we just show the Law, and show it well... then they will be better.
"It was a false, misleading dream, that God, His Law had given. That sinners could themselves redeem and by their works gain heaven" -- even a heaven on earth. "The Law is but a mirror bright, that brings the inbred sin to light, that lurks within our nature." Natural law does nothing to deal with *our* fallen nature.
But, but, but if we just get good laws passed.... then they will be better.
"Trust not in princes, they are but mortal. Earth-born they are, and soon decay. Naught are their counsels at life's last portal, when the dark grave doth claim its prey. Since, then, no man can help afford, trust ye in Christ our Lord!" This world is sinful, through and through. All around us we see nothing but death. The Law will not change that - the Law does not give life.
But, but, but the law works as a curb! That will make them better.
Eh... perhaps. If they listened. But how does a curb work? Only with threats of punishment - that things will be bad if you transgress. And arguing from a perspective of "natural law" doesn't do that. Natural Law appeals to what is right... not to punishment.
The lost must be shown that they are lost - that by giving into their sinful desires they receive no peace, no comfort, no joy. That sin offers nothing but false promises that do not deliver. The thing about sin is that, while it sounds good, it is bad. Show the consequences... not the back story behind, the reasons why. Sinful, selfish man doesn't care about nature, what should be... he cares only for what he wants to happen to him.
And even then... apart from Christ, that only leads to a slightly more gentle crushing, a slower, slightly less painful death. Apart from Christ, we only give people moral morphine, dulling the pain as they remain dying people in a dying world.
"My own good works all came to naught, No grace or merit gaining; Free will against God's judgment fought, dead to all good remaining." Apart from Christ, people are dead. Even if they play nicely, even if they bother me less and less, they are still dead. And all their works, however nice, however moral, however seemingly in accord with natural law they are, come to naught.
Of course, this was the point of Luther's Heidelburg Disputation - point number one:
"The law of God, the most salutary doctrine of life, cannot advance man on his way to righteousness, but rather hinders him."
Or as the hymnist sings - "From sin our flesh could not abstain, Sin held its sway unceasing. The task was useless and in vain, Our guilt was e'er increasing. None can remove sin's poisoned dart Or purify our guileful heart - So deep is our corruption."
Law will not save the world. It might curb some things... but only when people are convinced that what they want is actually bad for them (at least when we have people voting on laws). Otherwise, it all comes crashing down.
(Note: This is cross posted at Four and Twenty Blackbirds, so if you are interested you may follow the comments there.