How Christianity spread across the Roman Empire

In AD 30 a Galilean rabbi was executed as a criminal outside Jerusalem, leaving perhaps a hundred and twenty followers, no money, no political influence and no institutional standing.

Within three centuries their movement was the official religion of the most powerful empire on earth.

How?

The arithmetic

Rodney Stark — a sociologist, not a theologian — modelled the growth and found that it requires no miracle of numbers, only relentless consistency. A rate of about 40% per decade, which is roughly 3.4% a year, produces this:

• AD 40: about 1,000 Christians
• AD 100: about 7,500
• AD 200: about 217,000
• AD 300: about 6,300,000 — roughly 10% of the empire

That growth rate is unspectacular in any given year. Compounded over 260 years, it is decisive. And note that almost all of it happened while the faith was illegal.

The infrastructure God used

The timing was not accidental, and Paul says as much: "when the fullness of time had come, God sent forth his Son" (Galatians 4:4).

Roads. Rome had built roughly 400,000 kilometres of road, 80,000 of them paved. For the first time in history a person could travel from Britain to Syria on maintained roads under a single legal jurisdiction.

Peace. The Pax Romana meant that from Augustus onwards, travel across the Mediterranean world was safer than it had ever been. Piracy had been suppressed.

One language. Alexander's conquests had left koine Greek as the common tongue from Italy to India — a shared language for a message that needed to cross cultures.

The synagogue network. The Jewish diaspora had communities in every significant city, already monotheistic, already reading the Scriptures, already containing Gentile "God-fearers" attracted to Jewish ethics but unwilling to be circumcised. Paul went to the synagogue first in every city, and it is precisely that group of God-fearers who converted in large numbers — they got the God of Israel without the barrier.

Paul's strategy

Paul's method was deliberate, not opportunistic.

Target the hubs. Philippi (a Roman colony), Thessalonica (on the Via Egnatia), Corinth (a commercial crossroads), Ephesus (the great port of Asia). Plant in a city and the faith travels outward on the trade routes without you.

Plant and leave. He typically stayed months, appointed local elders, and moved on. He was building something that did not depend on him.

Aim at unreached ground. "I make it my ambition to preach the gospel, not where Christ has already been named, lest I build on someone else's foundation" (Romans 15:20).

Stay in touch. The letters we now call Scripture were pastoral follow-up, written to churches he could not revisit.

Persecution did the opposite of its intent

The first great scattering was not planned. After Stephen was stoned, "there arose on that day a great persecution against the church in Jerusalem, and they were all scattered throughout the regions of Judea and Samaria" — and Luke adds the sentence that explains everything: "now those who were scattered went about preaching the word" (Acts 8:1-4).

The persecutions came in waves. Nero (AD 64) scapegoated Christians for the fire of Rome; Tacitus describes them covered in animal skins and torn by dogs, or set alight to illuminate Nero's gardens, and notes that even Romans who despised them felt pity. Domitian, Trajan, Marcus Aurelius, and then the systematic empire-wide attempts: Decius (249–251), requiring every citizen to sacrifice and produce a certificate, and Diocletian (303–311), the Great Persecution — Scriptures burned, churches demolished, clergy imprisoned, believers executed en masse.

It failed. In 311 the dying Galerius issued an edict of toleration effectively admitting defeat.

Tertullian had written a century earlier: "the blood of the martyrs is the seed of the church." It sounds like rhetoric. It was an observation. Public executions of people who forgave their executioners were, in effect, the most powerful evangelistic events of the era, and they were staged by the state at its own expense.

What actually persuaded people

Here is where secular historians and Christian ones largely agree, and it is not what most Christians assume.

Two plagues. In AD 165 and again in AD 251, epidemics swept the empire, killing perhaps a quarter to a third of the population in affected cities. The standard response was flight — the physician Galen left Rome, and pagan families commonly abandoned sick relatives in the street rather than risk infection.

Christians stayed. They nursed their own and they nursed their pagan neighbours. Basic care — food, water, warmth — raises survival rates dramatically even without medicine, so Christian communities lost fewer people and, crucially, so did the pagans they nursed. Dionysius of Alexandria described believers who "died in their stead" caring for others.

The emperor Julian, who tried to restore paganism in the 360s, wrote in exasperation to a pagan priest that "the impious Galileans support not only their own poor but ours as well." He could not compete, and he knew why.

The treatment of women, children and the poor. Christians refused infanticide and the exposure of unwanted infants — overwhelmingly girls — and raised abandoned children. They opposed abortion, which was dangerous to mothers. They honoured widows. As a result their communities had more women, higher birth rates and lower mortality, which compounds over generations. Stark calculates this demographic effect alone accounts for a significant fraction of the growth.

Genuine social equality. In a rigidly stratified world, "there is neither Jew nor Greek, there is neither slave nor free, there is no male and female, for you are all one in Christ Jesus" (Galatians 3:28) was socially astonishing. A slave could be an elder over his master.

The willingness to die. People noticed that these were not fanatics seeking death — they generally avoided it where they honourably could — but that when it came they did not recant, and often forgave.

Constantine, and what it cost

In 312, on the eve of battle at the Milvian Bridge, Constantine reported a vision. He won, and in 313 the Edict of Milan granted Christians legal toleration. In 380 Theodosius made Nicene Christianity the empire's official religion.

Christians rightly celebrate the end of persecution. It is worth being clear-eyed about the rest of the bargain.

Legality became privilege, and privilege attracted people whose motives were career rather than conviction. The church acquired wealth, buildings and imperial politics. Bishops became officials. And within a few generations the state that had once executed Christians for heresy began executing heretics on the church's behalf — an inversion that should trouble anyone who has read the Gospels.

The church survived the persecutions intact. Its harder test was surviving success.

The pattern that has held

"You will receive power when the Holy Spirit has come upon you, and you will be my witnesses in Jerusalem and in all Judea and Samaria, and to the end of the earth" (Acts 1:8).

Ordinary people, telling other people, in the places they already were, and behaving in a way that made the message plausible. That is the whole mechanism. There was no strategy, no funding and no platform.

Within thirty years Paul could write that the gospel was "bearing fruit and increasing in the whole world" (Colossians 1:6).