b.c., the roman empire has been ruled by pagans. then, in 312 a.d., helena's son, constantine, has a dramatic change of faith. >> the story goes that on the eve of a great battle outside rome, he has a vision. >> he sees a cross of light in the sky. and he hears a voice that tells him by this, you will conquer. he takes it to mean that by the symbol of the cross and by christianity, he's going to become the lone emperor and the supreme figure in the known world. >> he wins the battle and becomes the first christian emperor. >> christianity is very much a minority cult, practiced probably by only about 10% of the population, if that. >> it's almost impossible to underestimate the impact of constantine's conversion to christianity. before that, the empire had been