Some proponents of 2k invoke natural law as an alternative to OT ethics and/or NT ethics. One of the problems with this appeal is that while there are several pretty clear prooftexts for natural revelation, prooftexts for natural law are sparse and problematic.
Keep in mind that I don’t object to philosophical arguments to support Christian ethics. I’m just dealing with the exegetical grounding for natural law.
The locus classicus is Rom 2:14-15. But as a prooftext for 2k, that’s dubious on several grounds.
i) To begin with, it’s far from clear that this passage has reference to unbelievers. For one thing, 2:15 alludes to Jer 31:33. This figures in the famous prophecy of the new covenant.
It would be self-defeating, from a 2k perspective, if the terms of the new covenant were inscribed on the hearts of unbelievers, for that would a type of special revelation, not general revelation. That would collapse the 2k distinction between the spiritual kingdom and the civil kingdom.
Of course, that interpretation is disputed. But 2k proponents can’t take their own interpretation of Rom 2:14-15 for granted. They must argue for their interpretation, and they must do so, in part, by arguing down the rival interpretation.
ii) Apropos (i), if the terms of the new covenant are directly implanted in the hearts of believers and unbelievers alike, or even that’s confined to believers alone, then that becomes a prooftext, not for 2k, but Quakerism. The doctrine of the inner light.
In that case, you don’t need special revelation either inside or outside the church. Christians don’t need the NT. For they have the content of the new covenant directly internalized. So that argument either proves too little or too much.
Of course, 2k proponents would say it has reference, not to believers, but unbelievers. However, that interpretation brings us back to the conundrum of why Paul would see Jeremiah’s oracle of the new covenant fulfilled in the hearts of unbelievers.
And remember that, from a 2k viewpoint, the unbelievers in question are not pre-Christian unbelievers who will undergo conversion. Rather, the 2k interpretation must regard the referents as unbelievers in contrast to believers, to maintain the contrast between the civil kingdom and the spiritual kingdom, general revelation and special revelation.
iii) Finally, inscribing the law on the heart is figurative language. So we must ask, what’s the literal truth which that picturesque metaphor signifies?
One explanation is that inscribing the law on the heart is a synonym for a related figure of speech: circumcision of the heart. God will give members of the new covenant a new heart.
That interpretation is reinforced by Jer 32:39: “I will give them one heart and one way, that they may fear me forever, for their own good and the good of their children after them.”
But if that’s correct, then what the passage denotes is not moral intuition, but regeneration.
2k proponents can, of course, take issue with one or more of these objections, but as it stands, they are skipping over key assumptions in their appeal. That’s not a given.