Witches—and the trials that made them famous—were tardy as far as supernatural scourges go, not medieval but early modern. It wasn’t until nearly the turn of the 15th century that Catholic Church scholars at the University of Paris proclaimed witchcraft “real,” the product of pacts with the devil. Before that, they struck a remarkably modern pose: Acts traditionally associated with witchcraft—black sorcery, night flights—were the stuff of overactive imaginations.

Even after deciding that witches might be real, it was almost another 100 years before the Church decided that doing something about them might be worth the bother. Until the 16th century, in the minds of most Christian authorities, witches took a back seat.

The Church eventually saw witches in droves and, to protect citizens against the perilous threat they posed to public safety and wellbeing, they had to be prosecuted and punished wherever they were found. The result was a literal witch hunt across Christendom. The hunt didn’t end for another 150 years, and by the time it did, no fewer than 80,000 people had been tried for witchcraft, half of them executed.

So what explains the Church’s witch 180? How could everyone’s favorite Halloween ghouls go from being the occasional hallucination to the haunt of every Christian? The answer might surprise you: It was market competition.

Between 1517 and 1521, the influential work of a German priest that criticized the Catholic Church for corruption and religious abuses spread throughout Europe. That priest was Martin Luther, whose Ninety-Five Theses catalyzed the Protestant Reformation. Before him, the Catholic Church enjoyed a virtual monopoly on the supply of religion in Christendom; its share of the “market” was nearly total. There were the occasional upstarts, mind you—your Cathars, your Waldensians. But the Church suppressed these minor competitors fast and easily, literally eliminating them from the marketplace with violence, Inquisitions, and crusades.

Violence is an effective way to maintain market share amid competition if you can get away with it. But often you can’t, which is the position the Church found itself in after Luther. Protestantism changed hearts and minds in the span of just a few years. Not only did many citizens in Christendom abruptly stop taking their religious cues from Catholic luminaries and start taking them from Protestant ones; many rulers did too.

And therein lay the Church’s problem. It tried using violence to suppress Protestant competition. But it’s hard to convince a Lutheran prince to conduct an Inquisition against himself and his fellow converts. Unable to forcibly prevent citizens from switching religious “brands” with the help of rulers, the Church tried a different tact: to woo them. And to do that, it began to advertise. Just as you’d expect in a marketplace buzzing with competition, the Church’s competitors—Protestant authorities—quickly followed suit.

What were the competing religious producers advertising? Their religious brand’s superior commitment and power to protect citizens from worldly manifestations of Satan’s evil. They did that by more energetically trying witches publicly. Similar to how Republicans and Democrats compete for undecided voters, Catholic and Protestant officials focused witch trial activity in religious battlegrounds during the Reformation and Counter-Reformation to win the loyalty of undecided Christians.

Popular belief in witchcraft made it easy. Although Christian authorities showed little interest in witches before the early 16th century, ordinary citizens had been complaining about them—“suspicious” neighbors or other outcasts in their communities—since the Middle Ages. Before the Reformation, when the Catholic Church enjoyed its religious monopoly, and in a few parts of Europe, such as Italy and Spain, where it continued to dominate the market even after, the Catholic Church had no reason to respond to public calls for witch trials. What would have been the point of prosecuting people for witchcraft when there was no benefit? Protestant entry into the religious marketplace, however, gave citizens a choice—and if you want to keep customers who have choice, you’d better give them what they want.

The rest is history—bloody history, wherein tens of thousands were burned at the stake. Religion works in unexpected ways. Sometimes, it turns out, market competition does too.

Peter T. Leeson is the Duncan Black professor of economics and law at George Mason University and author of WTF?! An Economic Tour of the Weird.