When Jesus He was young, another boy ran into him on the town street. Jesus cursed him – “You will go no further on your path” – and the boy fell dead. His parents complained about Jesus to Joseph and Mary: “Teach him to bless and not to curse; because he kills our children.” This was a slight exaggeration – previously Jesus had simply paralyzed another child – but the plea did not work. Jesus also cursed them and they became blind.

This is not the meek and gentle Jesus of the modern Church, but the Jesus of the “Infancy Gospel of Thomas”, one of the so-called apocrypha. Here, Catherine Nixey begins her fascinating new book, Heresy, by exploring the diverse religious landscape of the early Roman Empire, in which what ended up being orthodox Christianity was just one trader in a bewilderingly crowded market.

In the first three centuries AD, there were almost countless different types of Christians. There were those who denied the virgin birth, the divinity of Christ and the Resurrection. Others thought that the Holy Spirit was feminine, or that the mother of God was responsible for Creation. Some worshiped Jesus alongside “Pythagoras, Plato, Aristotle and the rest,” while another group worshiped him in the form of a snake. Exhaustingly, a secret cult elevated sex to an act of devotion: walk around the track twice with 365 women in full view of the congregation and you can announce “I am Christ.” He wasn’t the only child killer in the Infancy Gospel of Thomas, either. There were other “bad Jesuses,” like the one who sold one of his disciples into slavery, or the one who replaced another victim at the Crucifixion and watched laughing while the man died.

There were so many different sects, all claiming to be Christian, that modern scholars now tend to refer not to “primitive Christianity,” but to “Christianities.” The sources of these “other” Christianities are mostly obscure and difficult, and fall into three broad categories.

First, the pagans, such as Celsus, Porphyry, and the emperor Julian, who wrote works attacking Christianity; mostly these survive only as fragments, cited in refutation by Christian writers. Orthodox Christians, such as Irenaeus, Tertullian, and Eusebius, spent much energy denouncing other Christians whom they considered heretics; Obviously they couldn’t be more biased.

Catherine Nixey, author of Heresy

Catherine Nixey, author of Heresy

And then there are the heretics themselves, including texts discovered at Nag Hammadi in Egypt, numbering some 40 surviving gospels that were once labeled “apocryphal” as Orthodoxy fortified its borders. Nixey handles all this diffuse and refractory material with poise and good sense.

These versions of Christianity not only competed with each other. The new pagan cults of Mithras and Jupiter Dolichenus were spreading throughout the empire from the east. Paganism had no shortage of divine miracle workers. Apollonius of Tyana healed the sick and raised the dead. Once even an emperor joined in: in Alexandria, Vespasian was said to have made a lame man walk and a blind man see.

So why did Christianity win and why that specific brand? Scholars have long abandoned the idea that Christianity met people’s emotional or spiritual needs unlike paganism, and that its appeal to the poor and oppressed did not make it a state religion. In truth, it could be due to a series of fortunate historical accidents.

Aside from Nero scapegoating Christians for the Great Fire of Rome in 64 AD, all pagan persecutions were “from the bottom up” until the second half of the 3rd century AD: as Tertullian said: “If there is a famine “If there is a plague, the cry is immediately: ‘Christians to the lion.'” The Roman authorities would intervene.

But then came the barbarian invasions, civil wars, and short-lived emperors of the “3rd century crisis.” Something had gone wrong throughout the empire, and also with the pax deorum, Rome’s dealings with the traditional gods: we do right by you, you do right by us. Two emperors decided it must be Christians and ordered an empire-wide persecution. Instead, the fate of both men gave Christianity a shot in the arm: Decius was the first emperor killed in battle by barbarians (AD 251); Valerian was the only emperor captured alive by them (260 AD). And then, before the Battle of the Milvian Bridge (312 AD), Constantine had a vision in the sky: a cross of fire and the words “By this sign you will conquer.”

In itself, this was not so unusual. The Emperor Aurelian had had a vision of Sol Invictus before the Battle of Emesa (AD 272), and earlier Constantine himself had had a vision of Apollo. What was unusual, as Nixey ably points out, was the longevity of Constantine (died 337 AD) and his children (Constans II, died 361 AD). Half a century of imperial patronage and coercion would entrench the particular brand of Christianity favored by Constantine and his family.

So, only in retrospect is the triumph of that “orthodox” Christianity inevitable. With style, wit, and impressive erudition, Nixey shows how, “if history had been tilted just a little,” everything could have been very different. Heresy illuminates a forgotten world and is an absolute pleasure to read.

Picador publishes Heresy for £25. To order your copy for £19.99, call 0844 871 1514 or visit Telegraph books

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By Sam