Since it was a crime to be christian in the Roman Empire, the rulers and others would crucify those who were Christians. Many rulers made it a policy that people who were Christian would be killed and or ruined for the rest of their earth life.
Constantine had notice how God loved his people so he made it legal with the Edict of Milan. It was a policy which made and tolerated all religions especially Christianity. Constantine had beat all of his opponents for the throne and became Emperor.
The Roman Empire fell because of force and violence. Many people fought for the power and throne, they only ruled for about 4 years until they were assassinated from the senators and or invaders. Money became less valued and the German invaders had corrupted the government from the inside.