Diarmaid MacCulloch, a historian at Oxford, opens his six-episode account of Christianity's history by tracing the faith back to its start inside the Roman Empire, a small Jewish sect that grew into the religion of an empire within three centuries. He travels to the sites where that story unfolded, walking through ruins, churches, and archives to show how a persecuted movement negotiated with Roman power and eventually inherited it. MacCulloch narrates on location rather than from a studio, using each place, a catacomb, a basilica, a stretch of road, to anchor a specific stage in the argument he is building across the series: that Christianity's beliefs took shape through centuries of political compromise as much as theology. This first episode sets up the pattern the later ones follow, moving from doctrine to power and back again. It ends with Christianity established as an imperial religion, the position from which the rest of the series will follow its spread and its splits.