Michael Portillo investigates the legacy of the Roman Emperor Constantine - the man who transformed Christianity from a clandestine handful of followers of Jesus Christ into one of the world's great religions with a global reach of over two billion worshipers.

In 312AD Constantine is believed to have had some sort of religious epiphany or vision that converted him from Paganism - the traditional faith of Roman Emperors - to the new faith of Christianity. This transformed a persecuted cult into a religion of power and privilege within the Empire - with Constantine as head of both state and church.

Michael Portillo, a lapsed Catholic himself, uses his politician's mind to unravel the process by which Constantine and the church came together and questions how ruthless imperialism came to be reconciled with Christ's pacifist, altruistic values.

Portillo's conclusion is that once enthroned in a position of power, the church never looked back - and this has been a disaster for Christianity because 'power is for politicians and not for churchmen'.