What idea of natural law would be consistent with Ephesians 2:1-3’s description of natural man?
And you were dead in the trespasses and sins in which you once walked, following the course of this world, following the prince of the power of the air, the spirit that is now at work in the sons of disobedience— among whom we all once lived in the passions of our flesh, carrying out the desires of the body and the mind, and were by nature children of wrath, like the rest of mankind. (ESV)
In this light, Christians must hold either that (1) natural man lacks the ability to know or follow the natural law, or (2) natural man's following the natural law is consistent with being sinful. To put it another way, if man’s reason and will are sufficient for lawful actions via natural law, then either (1) natural man lacks reason/will or (2) the kind of lawful actions involved in following natural law are consistent with being dead in trespasses and sins, i.e. natural law does not lead to righteous acts.
A more severe condemnation of mankind could not have been pronounced. What does [Paul] leave to us, when he declares us to be the slaves of Satan, and subject to his will, so long as we live outside of the kingdom of Christ? …
Where is now the free-will, the guidance of reason, moral virtue…? What will they find that is pure or holy under the tyranny of the devil? …there is no obscurity in the apostle’s language; and all men who live according to the world … are here declared to fight under the reign of Satan.
Calvin seems to grant that will, reason and moral virtue are related, as do proponents of natural law. What he denies is that men can will, reason and attain moral virtue while they are under the tyranny of Satan. He denies, not that men can know natural law by reason, but that natural man can reason or will truly while he remains dead in sin.
In other words, a Christian idea of natural law cannot be determined simply by associating human reason with moral virtue; it must also take a position on whether human reason and will operate naturally before regeneration in Christ.