When it come to moral reasoning, the premises must encompass and contain the entire moral world. We are creatures, which means that our starting point must be bounded and finite, with a consistent set of moral assumptions embedded.
“The ultimate ethical injunctions have always been premises, never conclusions. Kant was perfectly right on that point at least: the imperative is categorical. Unless the ethical is assumed from the outset, no argument will bring you to it” (C.S. Lewis, Essays, p. 313).
This means that in order to presuppose Scripture (which I do), I must also presuppose the world in which it was given. In order to presuppose the morality taught in Scripture, which I do, I must also presuppose that same moral universe for the men to whom the Scriptures first came.
Before the first special revelation was given, the men who inhabited that world were men inhabiting a world about which the coming special revelation would be true. Another way of putting this is that men were without excuse before Romans 1 was written.
Natural revelation, natural law, is manifest. It is plain, obvious, clear. There is nothing obscure about it. More than this, it is inescapable. No man ever revolted against the divine order of things except by caricature — suppressing some features of the traditional morality and exaggerating others. We are incapable of inventing a new morality de novo. The most we can do is rearrange the furniture.
Christians who deny natural law, therefore, despite their best intentions, are doing this very same thing. An unbeliever might suppress certain elements of sexual morality and exaggerate our duty to “be nice, not mean,” for example. This is arbitrary cherry picking. But a Christian who says that the Bible is for those who believe in the Bible, and those who do not accept what the Bible says are not bound by it, is doing the same thing. He is suppressing one aspect of the only morality that exists and is exaggerating another part of it. In this case he is suppressing the necessary element of universal authority and exaggerating the principle of voluntarism. Different cherries are being picked, but everyone is in the same orchard. Every man has his own basket, but it is absurd to postulate a morality that can fit within a bushel. Nothing is moral unless it encompasses the entire orchard, not to mention the cosmos containing the orchard.
All men answer to God. All men answer to the same God, the living God. No aspect of the created order is outside His authority. The fact that this is the case is self-evident to every man. All of us know it. Unbelievers deal with this by stubborn and willful suppression of the detested truths. Compromised believers deal with it by retreat to a limited jurisdiction in order to flatter the demoted truth with false honors. “Jesus is Lord” is the monarch and emperor of all propositions, but when our retreat to commitment is complete, we find that we have made it the mayor of our little village — a village comprised of a handful of cottages. You know the cottages, our statement of faith, our constitutional documents, and our hymnal. Some of the hymns have a thatched roof, and are quite picturesque.