Those in power were especially alarmed at Christianity's growth within the Empire. Whereas those of the Jewish faith tended to keep to themselves, Christians were actively promoting and spreading their belief. The excellent road system, shipping routes and general peace of the Empire allowed Christian missionaries to spread their word relatively quickly. There was a constant hostility toward the religion, sporadically bursting into state organized violence as in the persecutions lead by Emperors Nero, Domitian, Marcus Aurelius, Decius, Gallus, Valerian and Diocletian. Noted victims include Peter and Paul who were arrested on separate occassions and executed in Rome sometime around 65 -67 AD.
Initially more popular with the poor and underprivileged, Christianity was slow to enter the ranks of the Roman army. The New Testament of the Bible mentions several occasions were Roman soldiers were convinced by Jesus. For instance, Matthew 8:5 tells of a Roman centurion embracing the new faith. In following years, there would be Christians found in the Roman army. however, the number was very small. The army placed a strong emphasis on pagan ritual, something quite distastefull to devout Christians. A Christian would have very little reason to join the ranks of an establishment so often employed in the persecution of his fellows, and there would have been great pressure discouraging exisiting soldiers from taking up the religion.
The Roman Empire went through a dramatic change with the rise of Emperor Constantine. On October 28, 312 AD Constantine's army defeated and killed his rival, Maxentius, at battle of Milvian Bridge. The night before the battle, Constantine was commanded in a dream to place a sign of Christ on the shields of his soldiers. Twenty-five years later Eusebius, the early Christian historian, gives us a far different account in his "Life of Constantine" When Constantine and his army were on their march toward Rome they observed in broad daylight a strange phenomenon in the sky: a cross of light and the words "by this sign you will be victor." During the next night Christ appeared to Constantine and instructed him to place the heavenly sign on the shields and standards of his army. This new symbol became known as the labarum. It consists of the overlapping of the "Chi and Ro" (the first 2 Greek letters in the word "Christ") forming a cross shape.
Attributing his success to his newfound faith, he made Christianity the official religion of the empire. Constantine ordered the end of any religious persecution in all of the Empire, a step he had already taken in his own provinces of Britainia and Gaul in 306. He proclaimed the Edict of Toleration at Milan in 313, in which Christianity was made legal throughout Rome. By 324 Constantine was in full control of a united empire. He relocated the imperial headquarters to Byzantium, whose name he then changed to Constantinople. The top political and military posts were now filled by Christians. Although they continued to operate, the old pagan temples were stripped of their former wealth, which was then shifted to Christian churches. The many fledgling churches across the empire thus acquired great strength and prosperity. In the East, the city of Constantinople would grow to be the capitol of a Christian Byzantine Empire, flourishing long after the Western Empire was lost to invading barbarians. Although no longer part of a unified Roman empire, western Europe would see the continued growth of Christianity.
It was in 337 AD that Constantine received Christian baptism on his deathbed. His chosen religion continued to grow eventually completely extinguishing the old pagan religion. It has been argued that Constantine was merely using the growing influence of Christianity for his own benefit. Whether or not this is true, it is agreed that the religion benefited tremendously from his patronage.References:
1. Late Roman Infantryman AD 236 - 565, by Simon MacDowall
2. Romano-Byzantine Armies 4th-9th Centuries, David Nicolle
3. The Holy Land, by Peter Connolly