If someone in your life does CrossFit, the high-intensity fitness training favored by Marines and first responders, you’ve ribbed them (out loud or in your mind) about being part of a cult. This is a natural response to their relentless insistence that CrossFit has changed their life, and that it will change your life, too! “The first rule of CrossFit,” the quip goes, “is always talk about CrossFit.”

So why are CrossFitters so wrapped up in this movement that’s grown from one fringe gym in a California industrial park to 10,000 independent “boxes” and the CrossFit Games, an international competition with over 200,000 participants? It’s not a cult of personality, although CrossFit founder Greg Glassman can rattle sabers with the best of them. There’s not much celebrity glitz-and-glamour. The facilities where people do CrossFit are bare bones, often in converted auto body workshops and defunct manufacturing spaces. There are…