I have come as both historian and tourist, to examine how the decision of one emperor changed the history of Western civilisation and Rome itself, its impact on the fabric of the city and on the lives of its citizens, nobles and high priests. Abandoning paganism risked everything that Rome stood for, triggering confusion, violence, power struggles, and setting Popes and Emperors on a collision course. So what transformed the Holy City of Rome from the pagan heart of the Roman Empire to the capital of Christendom?