The first christian emperor of Rome was?

It was Augustus. His original name was Gaius Octavius. It changed to Gaius Julius Caesar Octavianus when he was adopted by Julius Caesar. After Caesar’s assassination he had himself called Gaius Julius Caesar Divi Filius (son of the divine Julius Caesar, Caesar was deified after his death) to strengthen his ties with Julius Caesar’s veteran troops. He changed his name again: Imperator Caesar Divi Filius. Then the senate gave his the honorary title of Augustus which comes from the verb augere (to augment) and meant ‘the illustrious one’ or ‘the venerable one’ and was a title of religious, rather than political authority. His full name became Imperator Caesar Divi Filus Augustus.