With three simple numbers you can forecast your worship attendance one year from today. It's quite easy, and surprisingly reliable.

But before we talk about how … let's consider why. Is there value in looking into the future? Or, as Marty McFly discovered, is it just plain trouble to mess with Father Time?

Actually, I am a firm believer in forecasting for the simple reason that if our forecast indicates a potential problem, we can do something to keep it from happening. For example, suppose your forecast indicated that your worship attendance would be down by 10 percent in one year. If you could do something to prevent that situation, wouldn't you? I hope so.

So, let's look at how we can cheat the calendar and peek into the future. It requires three numbers: your visitor volume, your visitor retention, and your back door.

Visitor volume—the number of visitors/newcomers at your church services, as a percentage of your total attendance. To calculate this, add all the first-time ...