Under your limitations of space you have not, of course, had opportunity to elaborate your "policy"—toleration of abuse. … A Christian of your view is, as we have seen, committed to the belief that all people who practice "divorce"—certainly divorce as it is now legalized—are misusing the human machine (whatever philosophical defense they may put up), as certainly as men who get drunk (doubtless with a philosophic defense also). They are injuring themselves, other people, and society, by their behavior. And wrong behavior (if it is really wrong on universal principles) is progressive, always: it never stops at being "not very good," "second best"—it either reforms, or goes on to third-rate, bad, abominable.
The last Christian marriage I attended was held under your system: the bridal pair were "married" twice. They married one another before the Church's witness (a priest), using one set of formulas, and making a vow of lifelong fidelity (and the woman of obedience); they then married again before the State's witness… using another set of formulas and making no vow of fidelity or obedience. I felt it was an abominable proceeding—and also ridiculous, since the first set of formulas and vows included the latter as the lesser. In fact it was only not ridiculous on the assumption that the State was in fact saying by implication: I do not recognize the existence of your church; you may have taken certain vows in your meeting place but they are just foolishness, private taboos, a burden you take on yourself: a limited and impermanent contract is all that is really necessary for citizens. In other words this "sharp division" is a piece of propaganda, a counter-homily delivered to young Christians fresh from the solemn words of the Christian minister.
Tolkien understood the stakes. The debate strikes at the heart of what it means to confess that the Christian faith is "true." As Tolkien wrote, no article of Christian morality is intended exclusively for Christians. Rather, the faith teaches us that submitting to the laws of our creator is the surest way to live reconciled lives with his creation. This is what we ought to mean when we say Christianity is true. We don't simply mean that it provides factually accurate information about the world or that it offers an authentic path to spiritual fulfillment for those who choose to follow it. We mean that Christianity gives an accurate accounting of the world in its fullness and that it instructs us in how we ought to relate to the world.
In writing to Lewis on these matters Tolkien would have been preaching to the choir. Which is precisely what makes this oft-quoted section of Mere Christianity so baffling. If it came from any other pen, the natural thing would be to point out that the presuppositions behind the author's analogy are faulty. The argument simply assumes that religious dogma is strictly personal and, therefore, ultimately relative. You have your practices and I have mine. In this view, religious teachings are not a true description of how to live well and justly in the world, they are just a set of suggested behaviors that followers of a religion should consider practicing. There is no necessary connection between a religious command and human flourishing. This is simply the modern view of religion: Religion consists of private devotional beliefs and (empty) public ritual.