In his great book, “How not to be wrong: the power of mathematical thinking,” Jordan Ellenberg explores the logic of mathematics.

It’s a great, easy read.

In the course of the book, one of the stories he tells comes from World War II.Here’s the question, he writes.

You don’t want your planes shot down, so you armor them.

But the armor makes the planes heavier, and heavier planes are less maneuverable and use more fuel.

So where do you put a limited amount of armor? The military had planes that came back from engagements, covered with bullet holes–mostly in the fuselage and in the fuel systems, very little in the engines. The initial thought was–armor where the bullet holes are.

Sounds reasonable, right? Mathematician Abraham Wald, a Jew who had emigrated from Germany to the United States and now worked with the war department, had a different thought.

You need to take into consideration the missing bullet holes.

He thought you should armor where the bullet holes weren’t.

The reason was simple: what they were seeing were the planes that survived.

Bullet strikes would be spread pretty equally across all planes, if all planes were considered. By definition, then, the places where the bullets were striking were not critical.

What about the planes that didn’t make it home? If a plane got hit in the engine, it went down.

So–armor the engines.

The point is that when you’re studying a problem, you have to think about the whole data set–both the data you have, and the data you don’t.

And how does this apply to churches? Consider the question: what kills churches? What causes people to leave the church? What causes backsliding? Are we “armoring” people in the church for the issues that most of the saints in the church testify to having survived…

while not seeing the things that “killed” the ones who left? Are we responding to the requests made, the testimonies of problems surmounted, the known issues? Are those the issues that actually make people abandon the faith? Leave a church? Cause a church to close? Is it possible that the issues a church survives as part of its maturing process are the “less critical” bullet strikes–but the things that really kill churches, and cause believers to leave the faith, are far more silent issues.

As Screwtape might write, Why bother with mortal sins when busy schedules, and the self-importance that comes with business, could do the trick? When a church closes, it might be worth going around and asking–why did it close? When someone leaves the church, it might be worth going around and asking–why did they leave? And considering whether others are leaving for the same reason.