This ability to zoom out is critical in our generation, when so many people are, like the Corinthians, challenging biblical teaching on sex and sexuality. I don’t know how you would challenge a church that had accepted incest among its members, or which passages of Scripture you would appeal to. My guess is that I would have zoomed in on a specific instruction, like “Do not have sexual relations with your father’s wife; that would dishonour your father” (Leviticus 18:8), and left it at that.

But in 1 Corinthians 5, as he so often does, Paul zooms out. He starts not with Leviticus but with Exodus. He begins with gospel rather than with law. He heads for the defining event of the Old Testament Scriptures—the escape from Egypt by means of the Passover—and uses it to show why the church must not be compromised and defiled by unrepented immorality in its midst. (I say “unrepented” because it is important to remember that the problem here is not just the sin but the fact that everyone has responded to it with pride rather than grief. If this man, after having sex with his stepmother, was to repent of it and change his ways, Paul would be commending him rather than expelling him, and urging the church to welcome him back. This is exactly what happens in a different situation in 2 Corinthians 2:5-11.)

On the night that they escaped from Egypt, Paul explains, Israel ate unleavened bread. It was free from the fungus (yeast) which otherwise spreads throughout the dough and affects the whole loaf (1 Corinthians 5:6). At the very beginning of their journey to freedom, God gave Israel a meal to teach them that they were to be distinct from the world around them, and free from things that might otherwise infect God’s holy people and spread throughout the whole nation.

Well: the same is true of you, Paul says. Since most of us don’t think of yeast as a pollutant and quite enjoy bread with yeast in it, we might get a better sense of Paul’s picture if we paraphrased it as “a little mould spreads throughout the whole cheese”. Tolerating the mould, or the yeast, jeopardises the whole batch. The only way to save the cheese is to get rid of the mould.

And that, Paul is saying to the Corinthians, is what you must do with this incestuous man. You are a Passover people. You are called to be pure, undefiled, unleavened, and holy, and this is in fact what you already are. Christ himself has been sacrificed for you as a Passover lamb, pure and without blemish (v 7). So when you celebrate the “festival”—which I take to be the Lord’s Supper here—you must not be “leavened” with malice or evil, but be pure and “unleavened” with sincerity and truth (v 8). Otherwise the sin of this man, and your acceptance of it, will spread throughout the whole church like yeast through a loaf or like mould through a cheese, and you will be destroyed from the inside out.