Originally Posted by CaseyBessette

It seems to me that a fallen, unregenerate man discerning God's moral order through nature with such precision as to develop an entire ethic is to be distinguished from him being capable of enough knowledge of that order so as to be left inexcusable. Paul isn't saying that unbelievers have enough knowledge to create an ethical paradigm called "natural law," he's saying that what they do know is sufficient to leave them inexcusable.

It's worth thinking about whether or not the law is written on everyone's heart in the first place (since the fall, that is). In the beginning of Romans Paul says that "the works of the law" are written on their hearts, not the law per se. It seems to me that if you say the law is already written on the hearts of unbelievers (probably in some way this is true, but that's not what Paul says here), then one of the promises of the new covenant is meaningless:

Jeremiah 31:33. But this is the covenant that I will make with the house of Israel after those days, says the LORD: I will put My law in their minds, and write it on their hearts; and I will be their God, and they shall be My people.

It's by the work of the Spirit that the law is written on our hearts. Why would God make a promise for something that all unbelievers already possess? Because of the fall we lost knowledge, righteousness, and holiness -- and I believe included in that is our ability to grasp the law as it had been written on our hearts, at least to a degree.

But in Christ these things, by the work of the Spirit, are being restored. Paul connects these three (knowledge, righteousness, holiness) to the image of God, which presupposes that the image of God has at least in these three ways been defaced on account of the fall. The "knowledge" (Col. 3:10) is a pre-fall knowledge, and in the context it seems clear that "law" is a part of that knowledge being restored to believers.

At the moment, this is how I'm understanding all this.