How did the Romans help spread Christianity?

The Romans did not help spread Christianity. It was the other way round. Christianity was spread around the Roman Empire by the apostles, other missionaries and the clergy (when Christianity developed its churches). Christianity started in Judea, which was part of the Roman province of Syria; that is, it was part of the Roman Empire. It became widespread by the third century. It was persecuted several times. There were alternations of periods of toleration and periods of persecution. Then in the early fourth century, the emperor Constantine the Great promoted Christians in the imperial bureaucracy, tried to arbitrate between opposing Christian doctrines and built a number of important Christian Churches. In 380 mainstream Christianity was made state religion the sole legitimate religion and dissident Christian doctrines were branded as heretic and banned.

It can be said that Christianity is one of the main legacies of the Romans. It developed from a religion among a small group of Jews into a mass religion in the Roman days. It spread around the Roman Empire. It became the religion of the masses and then state religion. Catholic Christianity and Orthodox Christianity developed during the Late Roman Empire. They were originally called Latin or Western Christianity and Greek or Eastern Christianity respectively. The former was the main religion in the western part of the Roman Empire and the latter was the main religion in the eastern part of the Roman Empire.

The further spread of Catholicism in western Europe was promoted later by Charlemagne the king of the Franks when he developed the Carolingian Empire. The further spread of Orthodox Christianity was spread in eastern Europe by Greek missionaries and particularly by Photios, Patriarch of Constantinople, and the adoption of this religion in Russia by Vladimir the Great, the grand prince of Kiev.