Nero was the first Roman emperor to persecute Christians and he usually tortured them and then he brutally murdered them. Other Roman emperors like Trajan persecuted Christians. As a result of this Christianity grew even more and prospered.
Constantine was the first Roman emperor to become a Christian. He made Christianity legal in 313 A.D. with the Edict of Milan. He did not understand Christianity completely and started to discourage non-Christians and persecute them.
The fall of Rome happened because their government and leadership was really bad and their economy was horrible and Rome was split in the Western side and Eastern side. The Germanic tribes finally got Rome and conquered it . Western Rome was gone but the east side of the Byzantium empire was still prospering.