During the American Civil War, the relationship between poor sanitation and dysentery was not widely understood. As a result, more soldiers died of disease than of battle wounds. Indeed the cause-and-effect relationship is not well understood in much of the world today. Yet if the text of Leviticus often contrasts sharply with our modern sensibilities, it nevertheless does show from 3000 years ago a clear understanding of cause and effect.

The text recognizes that when people have a “discharge from the flesh” – diarrhea, most often – they are unclean. Their beds and their chairs are therefore unclean (Leviticus 15:4). Any person who touches them, or their beds or chairs, is unclean (15:5-7). If you’ve been in contact with such a person in any way, you have to wash your clothes and your body, and stay outdoors in the sun and not go back inside until evening (15:7, 10).

It would be silly to propose that they had developed the germ theory in ancient Israel. Their world view, their culture, their science was all quite different from ours; and indeed, some of the applications in the remainder of the chapter do seem quite odd to us today. Even so, we can see that they did understand that even if you can’t see it on the chair or the saddle, there’s something in the sick person’s discharge that can make you sick, and so you need to take very specific precautions to avoid getting that same illness yourself.

So we should want to be scrupulous about this, avoiding even the smallest bit of uncleanness in order to prevent illness; and we can see the obvious parallel that we should want to avoid even the smallest bit of sin in order to prevent guilt. Alas, Christians today often dismiss or mock that scrupulousness as legalistic or pharisaical. We assume instead that it is human nature to sin plenteously and it is God’s job to forgive us. But there is an important-but-neglected virtue in the deep desire for purity before the Lord: and we have mostly lost that.

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Have mercy on me, O God, according to your steadfast love. … Wash me thoroughly from my iniquity, and cleanse me from my sin. … Purge me with hyssop, and I shall be clean; wash me, and I shall be whiter than snow. … Blot out all my iniquities, and create in me a clean heart, O God.