Constantine did not convert the empire. He converted its incentives — and it still took eighty years and a different emperor before Nicene Christianity was legally required. Scrub the timeline to watch a persecuted minority become an established church. Then switch a privilege off and see which one was actually doing the work.
Drag the year240–420 CE
312CE
Legal standingMoney, buildings and staffCoercion against dissent
Constantine’s levers — switch one off
Every one of these is a law, not a sermon. Turn one off and rerun the century.