In 1553, in the city of Geneva, the theologian Michael Servetus was put to death by being burned at the stake. What had he done to be sentenced to this horrible fate? Was he a murderer? A rapist? A thief? He was none of these things: Servetus was put to death for suggesting that the doctrine of the trinity—the belief that the Christian God, Jesus Christ, and the Holy Spirit are all united as one being—was untrue.

What is the trinity? In the sixteenth century, the three largest branches of Christianity in western Europe (the Catholic Church, the Lutheran Church, and the Reformed Church) all held that God, Jesus, and the Holy Spirit were united in one being. The official doctrine was that all three were divine, but that couldn't mean that their church worshiped three different gods—somehow these figures were three and one at the same time. The city of Geneva (where the Reformed Church was in league with the city authorities) convicted Servetus of a capital crime for calling this belief into question. His doubt—and his attempts to talk to others about it—made him a heretic: someone who pretended to be Christian but really wasn't.