In the year 325, the first ecumenical council of the Christian Church convened in the city of Nicaea in Asia Minor. Bishops from all over the Christian world were summoned by the Roman emperor Constantine to debate matters of doctrine, and address what was thought to be a serious threat to the integrity of the Church. Arian, a prominent Alexandrian preacher, had been teaching his followers a novel and unique brand of theology.

The Church had up to this point maintained that Jesus Christ and God the Father, were of the same eternal essence—that is to say, that Jesus was as much God as the separate and distinct being called God. Arian and his followers, however, did not agree. They claimed that Jesus Christ was not equivalent to God the Father, but that he was His first creation; an important spiritual being to be sure, but a subordinate one. It was not the first time the budding Christian orthodoxy had been questioned, but Arian’s movement struck at the heart of Church philosophy. It needed to be addressed.

For almost a month, hundreds of bishops argued over the differences between the physis and hypostasis of God and Christ, and whether they should adopt the homoousian proposal or the homoiousian one. Outside the conference a fed-up Christian, frustrated by the supposedly learned discussions going on inside, begged for sanity. “Christ and his apostles did not teach us dialectics, art, nor vain subtleties,” he astutely pointed out, “but simple-mindedness, which is preserved by faith and good works.” Like most Christian heresies, the Arians did not dispute anything Jesus taught; it was mainly concerned with small, seemingly unimportant bits that meant little to the everyday Christian. Nevertheless, the devil, as they say, is in the details.

In Heretics, author Jonathan Wright aims to get to the bottom of why such idiosyncratic transgressions warranted the fierce and, at times, brutal response they were given, and show how such heresies paradoxically helped early Christianity define itself and cultivate a broader appeal. It’s a solid and entertaining book, particularly when illuminating the pre-Reformation scuffles of the early and medieval Church. The Reformation and Counterreformation are perhaps too large a topic for Heretics, though, and once the book enters the modern era, the lack of fire and brimstone makes for slower, albeit still enlightening reading.

Wright points out that the term “heresy” is derived from the Greek word for “to choose;” in the context of a religion that believes its doctrines are handed down by God, choice is a luxury that will not be tolerated. To allow for choice undermines institutional authority. This problem was compounded by Christianity’s relationship with the empires and kingdoms of Europe. When a religion becomes intertwined with a political entity, as it did in the late Roman and Byzantine empires, heresy becomes treason. Wright does an excellent job of charting the evolution of heresy and explaining why transgressions that were originally met with sternly-worded tracts and ominous, yet civil, council debates became cause for burnings at the stake and other violent persecutions.

In his chapter on the Reformation, Wright expresses surprise that the apostle Paul is often a touchstone for heretics, from Martin Luther all the way back to the Gnostic Marcion. “There is surely an irony,” he writes, “in the fact that a person who devoted himself to cultivating Christian unity inspired so many heretics.” This, however, ignores the fact that Paul himself was a heretic—not a Christian heretic, but a Jewish one. Paul was instrumental in separating early Christianity from its Jewish roots, arguing that the Mosaic Law that governed Jewish life (requiring kosher diets and circumcision, among other things) no longer applied to followers of Christ. His innovations were bitterly resisted by rival Christian sects, like the Ebionites, who clung to the old ways.

Paul differed from later heretics like Marcion or Arian in that his doctrinal innovations were aimed at broadening the appeal of Christianity. Pagans were easier to convert if they didn’t have to give up shellfish or endure an adult circumcision; coupled with the egalitarianism of early Christianity, these allowances turned the religion from a regional curio to a global blockbuster.

Marcion’s Gnosticism, in contrast, argued that a “divine spark” was present only in a select minority, and claimed the material world was utterly corrupt. The Montanists advocated for a strict, ascetic form of Christianity that stressed self-deprivation and discipline. Neither of these were the kind of pitch likely to win masses of converts.  Donatists said that Christians who had lapsed in the face of persecution could not be readmitted to the congregation. These heresies sought to limit the scope of the Church, to drive people out in an effort to cultivate a smaller, purer assembly of saints. The larger Christian Church stood to suffer by being associated with sects or movements that seemed exclusionary or needlessly onerous; these heresies were stifled because they threatened early Christianity’s greatest strength: its inclusiveness.

In modern times, it seems that even the Church has forgotten that inclusiveness can be a strength. Wright’s focus on heresies tells only a portion of the story of Christianity, but this focus may also help highlight the most important part of that story, and help readers better understand what the Church needs to do to survive into the future.