Christians were originally persecuted a lot in Rome. The religion wasn't very popular and they were often attacked and killed by government officials.
Luckily, the emperors kept growing less hostile towards the Christians until they outright ignored their very existence. That all changed with an emperor named Constantine.
Constantine's father was one of the less hostile emperors towards Christians, and his son looked to be a lot like his father until he went to war with one of his rivals, Maxentius. Reportedly, in a dream, he was shown triumphing over his enemy. In the dream, a voice told him to put the sign of the cross on his helmet and the shields of his soldiers. He ordered his troops to do that, and he won.
Because of this victory, the emperor made Christianity the official religion of Rome. He built many churches, destroyed temples for other gods, and gave tax exemptions to the bishops and everyone else who worked in churches.
Probably the most famous building Constantine built was the Haghia Sophia, a gigantic church.
His son, Julian, tried to rebuild the temples and reinstall paganism as the leading religion in the Roman Empire. The temples he built were razed shortly after they were constructed, and then Julian died. His son was a Christian emperor and all the emperors from then on followed in his footsteps.