“Because the Bible tells me so.”
Most of us think of that phrase as part of one of a beloved children’s hymns (“Jesus loves me this I know, for the Bible tells me so.”). But it’s also one of the most sophisticated premises for a moral argument. Because Scripture is a channel through which God’s self-revelation can be known, arguments based on moral appeals to the Bible (i.e., interpreted through proper contextualization and hermeneutical principles) should be particularly compelling and authoritative.
Unfortunately, this is rarely true when arguing with modern Christians, much less with non-believers. The problem is not with Scripture, of course, but with the reasoning abilities of the average person. Using Biblical arguments in moral discussions is often like using calculus to convince people who think arithmetic is akin to witchcraft: You first need to disentangle their confused worldview before they can even begin to understand.
That is why Christians often need to arm themselves with “translations” of moral appeals that are more comprehensible to people acclimatized to a culture of pluralism. “Secular” moral arguments are not better—often they lack the solid foundation of Biblical-based appeals—but for those who reject or can’t comprehend religious-based arguments, they can be more persuasive.
A good example is David Marcus’s argument in “The Amoral Case Against Paying For Sex.” Marcus agrees that we “need arguments against sex for pay that do not merely appeal to moral authority” and comes up with a clever and compelling comparison: prostitution is like predatory lending.