In my reading this past month, I discovered that Immanuel Kant made a case for a Christian sexual ethic but without using any appeal to the Bible or theology. In "Duties Toward the Body in Respect of Sexual Impulse" (Kant, Lectures in Ethics) he argued that sex outside of marriage dishonors human dignity. He reasoned that when you ask for sex without giving your whole self to the other person in marriage ("person, body and soul, for good and ill and in every respect"), you turn the sex partner into an object, a mere means to a selfish end, instead of an end in him (or her) self. Kant's famous "categorical imperative" was that human beings should never be treated as means, but only as ends. Using only this belief, which is intuitive for many modern people, he argued that you should never have sex outside of marriage.
I compared this with Wendell Berry (in Sex, Economy, Freedom and Community and other volumes) who also makes a case for the Christian sex ethic without appealing to overtly religious arguments or sources. Berry says that sex outside of marriage is sex for its own individual fulfillment rather than for building community. That, he argues, is a market-shaped, individualistic, consumerist approach to the human body. Instead, he insists, sex should be only used inside of marriage because there it becomes a nurturing discipline that establishes community, creating the deep stability between parents necessary for children to flourish.
What the two men have in common is that they both start with premises that most modern, secular readers share, but then they use those commonly held beliefs to drive them toward a Christian sex ethic, which has been largely abandoned by most secular people. They do this without appealing to the Bible or to other sources of religious authority.
Does this mean that it is possible to prove Christian morality is true without appealing to the Bible itself? No, I don't think so. Though Kant believed that reason was all you needed to discover ethical truth, his high view of human dignity still was ultimately a belief. It was not the inescapable conclusion of logic or empirical investigation. And Berry's appeal to the importance of community over individual freedom is also, in the end, a vision of human flourishing that can't be proved rationally. Berry's and Kant's arguments can't prove the Christian sex ethic to someone who doesn't accept their basic premise-beliefs. But if you share those beliefs, then their case is quite powerful.
Here's what I learn from Kant and Berry. First, there are ways to argue in public discourse for various features of the Christian account of human flourishing without directly appealing to Biblical texts or to God. For example, if I am a Christian in politics, and I am speaking to a body of people who I know will resonate to Kantian views of human dignity or Berryan views of community, then it is possible to make a compelling argument for practices that are rooted in Christian truth. Why? Because people without an overt religious profession still hold many true beliefs about human dignity or community that are spiritually "there" in their souls because they are created in the image of God. We should not be under the illusion that we can "prove" Christianity to secular people however. The compelling nature of our argument relies on discovering the underlying beliefs that a non-believer has that match up with Biblical truth. Only if they grant these beliefs can we make our case.
Second, I find it is often helpful even when preaching to briefly recapitulate arguments such as these from Kant, Berry, and others. Why? The ultimate foundation for what we believe as Christians is the authority of God's Word, but often the people we preach to are not convinced of the Bible's complete trustworthiness. Here is an example. I may first present what the Bible says about sexuality. Then I may briefly make a Kantian argument (which C.S. Lewis also makes inMere Christianity) about how sex outside of marriage de-humanizes or a Berryan one about how it harms community. Then I can add, "These are only some of the terrible results that come from violating God's design for sexuality. There are certainly many others." This approach both honors the Bible as the final authority for our lives and draws in listeners who, while not yet sure about the Bible's inspiration, share the premises of Kant, Berry, or whomever else you use.
I think that in our contemporary society, Christians' beliefs about sex and gender will be one of the biggest points of conflict with our culture. We will need to co-opt some of our culture's own baseline narratives (the importance of human dignity and community) in order to gain any hearing at all for our beliefs.