Making Christianity the dominant religion of Rome was not an easy task, since the Roman Empire was deeply rooted in its traditional religions, but Roman Emperor Constantine the Great did this within a remarkably short span of time.
The opinion is divided among scholars as to what influenced Constantine the Great to convert to Christianity himself. Some historians say that he received a message in his dream, while according to most historians he along with his entire army saw something or a symbol in the sky before the Battle of Milvian Bridge. This convinced Constantine I that the Christian God was with him, so he ordered his army to paint a Christian symbol on their shields and banners. After emerging victorious from this battle, Constantine became the emperor of Rome and had himself officially baptized as a Christian.
Following this, he took small steps to gradually spread Christianity in the Roman Empire. The baptism itself played a pivotal role in influencing most of the senior Roman officials to follow Emperor Constantine's lead as it put them in a better position to gain his favor.
Constantine the Great also became the patron of Christianity and built churches throughout the Roman Empire, especially in Constantinople, the newly established capital of ancient Rome. In addition to funding the construction of churches, he stopped the flow of money towards the old Roman temples, because of which the temples had to eventually close down. He even allowed people to raid the Roman temples and did noting to stop such activities.
Many rich Roman citizens and minor officials who were looking for a way to get into a position of power made use of this opportunity. The church was very well organized with an hierarchy and the people at the top were gaining some political favor. So, many of these opportunists gave large sums of money to the church as charity and got themselves a high position in the church hierarchy.
Slowly he brought reforms to all public offices and the legal system in light of Christianity. The practice of crucifixion and gladiatorial games were officially stopped. Christians were even given preference for positions of power. Towards the end of this reign, when Christianity has already become the dominant religion of the Roman Empire, Constantine the Great took a more direct approach and started destroying the old Roman temples.
Who would have though that a religion followed by a small group of Romans, that had been persecuted for more than two centuries since the reign of Emperor Nero, would one day become the official religion of Rome.