How did Christianity take over the Roman Empire?
✓Fact-checked & reviewed for Nicene orthodoxyHow we review →Not because of Constantine — and “Constantine did it” explains the wrong part of the story. His conversion (312) and the later establishment of Christianity (380) came after the church was already too large to stamp out. The empire's own attempts to destroy the church — under Decius, Valerian, and Diocletian — had already failed, and the state formally gave up first: the emperor Galerius issued an edict of toleration in 311, a year before Constantine's victory at the Milvian Bridge.
How many Christians were there? Nobody counted, so every figure is an estimate. Rodney Stark's model yields roughly six million by AD 300 — a little over a tenth of the empire — while Bart Ehrman's estimate is lower, around two and a half to three million. What is not in dispute is the shape: a small, illegal, politically powerless movement that grew steadily for nearly three centuries, its spread attested even by hostile witnesses like Pliny and Lucian.
Doesn't such fast growth prove Christianity is true? No — popularity is not a truth-detector, and other movements have spread fast and been mistaken. What the growth shows is narrower: the imperial-power story is false. The case that the faith is true rests on different evidence, above all the resurrection.