In this time of the Roman Empire, it was a crime to be a Christian. Many of the emperors pursacuted Christians. This pursacution only resulted in more people becoming Christians and people standing up for their faith.
Constantine believed that God helped him become emperor after he was victorious out of four other generals including himself. He made the Edict of Milan in 313 that declared that anyone could worship any god they wished. Constantine did not fully get Christianity, so he started persecuting in the name of Christianity.
The Roman Empire fell when Theodosius I split the empire between his two sons in 395. The Western Roman Empire quickly fell mainly to invading Germanic Tribes. It also fell because founded on force and violence, Rome could not hold to having Christianity, founded on peace, be its main religion.