How does a person get canonized? Or, to be more precise, what is it that distinguishes the 10,000 or so official Catholic saints from the rest of us 1.2 billion Catholic sinners?

On Wednesday, Pope Francis canonized Junipero Serra, an 18th-century Spanish missionary who introduced—or imposed—Catholicism among Native American communities in California. Emma Green elsewhere details the controversy surrounding Serra’s reputation as a cultural imperialist, and the impact of the missionary project as a whole on native communities. But Serra is controversial for another reason—he’s only got one Rome-certified miracle to his name. (A miracle, per the catechism, is “a sign or wonder, such as a healing or the control of nature, which can only be attributed to divine power.”) Saints are usually required to perform at least two, though martyrs are an exception to this rule and popes can make other exceptions, as Francis has done in this case.

It’s not easy to become a saint, though Pope John Paul II made it a little easier, and it can take a while. Serra died in 1794, and the one miracle Church authorities have confidently declared he performed was in 1960, when a nun in St. Louis was cured of a mystery disease, thought to be lupus, after having prayed to him on her deathbed. (Rejected miracles attributed to Serra: a non-injurious fall from a horse, a life-changing epiphany experienced at the holy man’s tomb, and a cured case of alcoholism.) Still, the rules for sainthood are pretty straightforward. Essentially, to become a saint, you must 1. be dead 2. have demonstrated “heroic virtue” while alive and 3. perform miracles posthumously.

But what counts as a miracle, and who gets to say? This question lies at the heart of why sainthood as such, beyond any particular contentious figure, has been controversial for centuries. In Medieval Europe, for example, holy men and women were thought to heal or even resurrect people from beyond the grave through the power of their relics, which were often parts of their bodies which believers preserved after their deaths. The Stanford sociologist Paolo Parigi documented what this looked like in his book The Rationalization of Miracles (which I helped edit). Relics were in such high demand that in 1589, when the locally revered Friar Raniero died in Borgo San Sepolcro, Italy, a mob of the faithful, some wielding scissors, rushed his body as it lay inside a church, cutting his hair, beard, and eyelashes, and removing his fingernails as well as one of his teeth. Objects collected thus could be, say, boiled in a broth and consumed to ease a difficult childbirth, or placed on a wound to soothe it.

The Protestant reformers of the 16th century tended to view this kind of thing as superstition and sorcery, Parigi told me. With Rome’s then-lax oversight of local devotion, it was also subject to fraud. The reformer John Calvin mocked the veneration of relics as “superfluous and frivolous” and asked: “If idolatry is just to transfer the honor of God to others, can we deny that this is idolatry?” The fact that fraudulent relics were proliferating in Europe meant, in Calvin’s words, that Catholics were committing the “double error” of not only worshiping body parts, but worshiping the “wrong” ones. He wrote:

It was said that in this city there was an arm of Saint Anthony. While enclosed in its case, all kissed and worshipped it, but when brought forward into view, it proved to be a nameless part of a stag.  On a certain great altar lay part of the brain of Saint Peter. So long as it was in its case, no man doubted … but when the nest was shaken up, and observed more accurately, it turned out to be a pumice stone.

If all Europe’s fake relics were gathered together, he surmised, each apostle would have more than four bodies, and each saint two or three.

Such criticisms helped galvanize reformers within the Catholic Church to tighten control over the process of declaring sainthood—to, in Parigi’s words, rationalize “the magic at the root of miracles” by developing strict procedures for who could become a saint, and how. Though Rome had formally been in charge of declaring saints since the 12th century, in practice local worshippers created their own saints unchecked for 500 years. Rome had no mechanism, nor any particular need, to enforce the rule; local practices posed no real threat to its authority. Beginning in the 16th century, the Reformation did pose such a threat; though in the field of sainthood, at least, it took the Church more than another century to figure out how to respond. What developed was the legalistic procedure—literal canonization trials, complete with canon lawyers to act as the “prosecution” arguing against a candidate—for validating miracles and centralizing sanctity that survived until John Paul II’s reforms in the mid-1980s.

Miracles, meanwhile, would never be the same, which can’t be blamed entirely on the Protestants. “In the most popular sense,” Parigi wrote, “a miracle is a statement about the boundaries of science”—and as those boundaries expanded to encompass more and more events, the space left for things only attributable to “divine power” shrank. Modern miracles are mostly healings, and the Vatican reviews them with both doctors and theologians to make sure they are “complete,” “instantaneous,” “durable,” and inexplicable but for the intercession of the holy person. Parigi surmises that the prevalence of healings, too, reflects a bureaucracy’s efforts to tame magic with rationality. It’s not like anyone’s claiming Junipero Serra flew across Northern California. “If you talked to somebody who received a healing, you may be skeptical but you think, well, perhaps. Who knows. … If you go with an approach that’s based on science, both things, the healing and the flying, are impossible. But one is more impossible than the other.”