The triumph of Christianity over the pagan religions of ancient Rome led to the greatest historical transformation the West has ever seen: a transformation that was not only religious, but also social, political and cultural. Just in terms of “high culture,” Western art, music, literature and philosophy would have been incalculably different had the masses continued to worship the gods of the Roman pantheon instead of the one God of Jesus—if paganism, rather than Christianity, had inspired their imaginations and guided their thoughts. The Middle Ages, the Renaissance and modernity as we know them would also have been unimaginably different.

But how did it happen? According to our earliest records, the first “Christians” to believe in the death and resurrection of Jesus were 11 male disciples and a handful of women—say 20 people altogether. These were lower-class, uneducated day laborers from a remote corner of the Roman Empire. And yet, within three centuries, the Christian church could count some 3 million adherents. By the end of the 4th century, it was the official religion of Rome, numbering 30 million followers—or half the Empire.

A century after that, there were very few pagans left.

Christians today might claim that their faith triumphed over the other Roman religions because it was (and is) true, right and good. That may be so. But one still needs to consider the historical contingencies that led to the Christian conquest, and in particular the brilliant strategy the Christian evangelistic campaign used in winning converts. These are five aspects of that strategy:

Strangely enough, Christianity did not succeed in taking over the ancient world simply by addressing deeply sensed needs of its target audience, the pagan adherents of traditional polytheistic religions. On the contrary, it actually created a need that almost no one knew they had.

Everyone in the ancient world, except for Jews, was “pagan”—that is, they believed in many gods. These gods—whether the state gods of Rome, the local municipal gods, the family gods, the gods of forests, mountains, streams and meadows—were active in the world, involved with humans on every level. They ensured that crops would grow and livestock would reproduce; they brought rain and protected against storms; they warded off disease and restored the sick to health; they maintained social stability; and provided military victories for the troops.

The gods would do such things in exchange for proper worship, which at all times and everywhere involved saying the right prayers and performing the appropriate sacrifices. If the gods were not worshiped in these ways—if they were ignored—they could bring disastrous retribution: drought, epidemic, economic collapse, military defeat and so on.

But the key point is that the gods were principally active—for good or ill—in the present life, to worshippers in the here and now. Almost no one in the Roman world practiced religion in order to escape eternal punishment or receive an eternal reward—that is, until the Christians came along.

Unlike pagans, Christians claimed there was only one God and that he should be worshiped not by sacrifice but by proper belief. Anyone who didn’t believe the right things would be considered a transgressor before God. And, most significant of all, rewards and punishments would be dispensed not only in this life, but in the life to come: either eternal bliss in heaven or everlasting torment in the fires of hell. Religion had never promoted such an idea before. Christians created a need for salvation that no one knew they had. They then argued that they alone could meet the need. And they succeeded massively.

Everyone in the ancient world knew that divinity was all about power. Humans cannot control whether it rains or an epidemic destroys the community or a natural disasters hits; but the gods can. They can provide for humans what mere mortals cannot do for themselves. This stood at the root of all ancient religion. And it became the chief selling point of the Christian message. Christians declared that their God was more powerful than any other god—in fact, more powerful than all the supposed other gods combined. God alone was God, and he alone could provide what people need.

The power struggle between the Christian and pagan gods is on full display in a wide range of ancient texts. Consider the apocryphal book called the Acts of John, an account of the missionary escapades of Jesus’ disciple John the Son of Zebedee. At one point in the narrative, John visits the city of Ephesus and its renowned temple to the goddess Athena. Entering the sacred site, John ascends a platform and issues a challenge to a large crowd of pagans: They are to pray to their divine protectoress to strike him dead. If she fails to respond, he in turn will ask his God to kill all of them. The crowd is terrified—they have already seen John raise people from the dead, and they know his God means business. When they refuse to take the challenge, John curses the divinity of the place, and suddenly the altar of Artemis splits into pieces, the idols break apart and the roof caves in, killing the goddess’s chief priest on the spot. The crowd makes the expected response: “There is only one God, that of John…now we have converted, since we have seen your miraculous deeds.”

Although obviously legendary, the tale conveys an important truth. Miraculous powers were the Christians’ evangelistic calling card, their compelling proof. Jesus himself, the son of God, had performed one miracle after the other. He was born of a virgin; he fulfilled prophecies spoken centuries earlier by ancient seers; he healed the sick; he cast out demons; he raised the dead. And if all that wasn’t enough, at the end of his life he himself rose from the grave and ascended to heaven to dwell with God forevermore. His disciples also did miracles—amazing miracles—all recorded for posterity in writings widely available. And the miracles continued to the present day. People became convinced by these stories. Not en masse, but one person at a time.

Christianity did not initially succeed by taking its message to the great and the powerful, the mighty Roman elite. It succeeded at first as a grassroots movement. The original followers of Jesus told those close to them what they believed: that the great miracle worker Jesus had been raised from the dead, and that his wonders continued to be performed among those who believed in him. They convinced others. Not most of those they talked with, but some. And as it turns out, small but steady growth from the ground up is all it took.

One might think that if Christianity went from some 20 people in the year of Jesus’ death, say 30 CE, to something like 3 million people 300 years later, there must have been massive evangelistic rallies, converting thousands at a time, each and every day. That wasn’t the case at all. If you chart the necessary rate of growth along an exponential curve, the Christian movement needed to increase at a rate of around 3 percent annually. That is to say, if there are 100 Christians this year, there need to be only three conversions by the year’s end. If that happens year after year after year, the numbers eventually pile up. Later in the history of the movement, when there are 100,000 Christians, the same annual growth rate will yield 3,000 converts; when there are 1 million Christians, 30,000 converts. In one year.

The key was to reach people one at a time. It grows from the bottom up, not the top down. The top will eventually convert. But you start below, at the base, where most people actually live.

Christianity succeeded in large measure because it required potential converts to make a decision that was exclusive and final. If they chose to join the church, they had to abandon all previous religious commitments and associations. For the Christian faith, it was all or nothing, so as it fed its own growth, it devoured the competition.

That may seem unusual by contemporary standards, since in today’s world we normally understand that someone who becomes Baptist cannot remain Buddhist; a Muslim is not a Mormon. But we ourselves accept exclusive religions precisely because the early Christians convinced the world that this is how it ought to be. Personal religion is one thing or another, not both—or several—at once.

The pagan religions didn’t operate like that at all. Since pagans all worshiped many gods, there was no sense that any one God demanded exclusive attention. Quite the opposite. Within pagan circles, if you chose to worship a new god—say, Apollo—that didn’t mean you gave up the worship of another, such as Zeus. No, you worshiped both—along with Hermes, Athena, Ares, your city gods, your family gods and whichever others you chose, whenever you chose.

Christians, though, maintained there was only one God, and if you followed him, you had to abandon the others.

In the long run, this meant that every adherent Christians gained was completely lost to paganism. No other religion demanded such exclusivity. For that reason, as Christianity grew, it destroyed all competition in its wake. And it went on like that for millennia, as Christians forged into new territories, toppling Celtic gods, Norse gods and many others.

Even though early Christianity was a grassroots movement, throughout its first three centuries it recognized fully the importance of converting influential supporters. At the beginning, this simply meant converting an adult male who was head of his household—the paterfamilias. In the Roman world, the paterfamilias chose the family’s religion. If you converted him, you got his wife, children and slaves in the package. Even if it was a small family—a husband, wife and two children—the conversion of one person meant the conversion of four. That multiplier effect went a long way toward achieving the needed 3 percent annual growth rate.