How did the conversion of Constantine influence Christianity?

It was October, 312. A young general who had the allegiance of all the Roman troops from Britain and Gaul was marching toward Rome to challenge Maxentius, another claimant to the imperial throne.

As the story goes, General Constantine looked up and saw a cross of light in the sky. An inscription read, “In this conquer.” The superstitious soldier was already beginning to reject the Roman deities in favor of a single god. His father had worshiped a supreme sun-god. Could this be a favorable omen from that god on the eve of battle?

Later, Christ appeared to Constantine in a dream, bearing the same sign, a cross with the top bent over, resembling the Greek letters chi and rho, the first two letters of Christos. The general was instructed to mark this sign on his soldiers’ shields. He did.

As promised, Constantine won the battle.
It was one of several decisive moments in a quarter century of violent change. If you had left Rome in A.D. 305, to spend twenty years in the desert, you would have returned expecting to find Christianity dead or facing the final deathblows of persecution. Instead, Christianity had become the favored religion of the empire.

After Diocletian, one of the most brilliant Roman emperors, had taken power in 284, he began a massive reorganization that would shape up the military, the economy, and the civil service. For quite a while, he left the Christians alone.
One of Diocletian’s brainstorms was the restructuring of imperial power. He divided the empire into East and West; each side would have an emperor and a vice-emperor (or caesar). 

Each emperor would serve twenty years, and then the Caesars would take over for twenty years, and so on. In 286, Diocletian appointed Maximian as emperor of the West, while he himself continued to rule the East. The caesars were Constantius Chlorus (father of Constantine) in the west and Galerius in the east.

Galerius was strongly anti-Christian. (He reportedly blamed the loss of one battle on a Christian soldier who crossed himself.) When the Eastern emperor took anti-Christian positions, he probably did so at Galerius’ instigation. It was all part of the reorganization of the empire—so the logic went: Rome had a uniform currency, a uniform political system, it should have a uniform religion. Christians were in the way.

Beginning in 298, Christians were rooted out of the army and civil service. In 303, the Great Persecution started. Authorities planned a crackdown on Christians to begin at the Feast of Terminalia, February 23. Churches were razed, Scriptures seized, and services prohibited. At first there was no bloodshed, but Galerius soon changed that. When Diocletian and Maximian resigned (according to schedule), in 305, Galerius unleashed a fiercer persecution. Constantius, who ruled in the West, was generally lenient. But horror stories from the East are plentiful. Continuing through 310, the persecution took the lives of many Christian martyrs.

But Galerius was unable to crush the church. Strangely, on his deathbed, he changed his mind. In another great moment, April 30, 311, the vicious emperor gave up the fight against Christianity, issuing the Edict of Toleration. Always a politician, he insisted that he had done everything for the good of the empire, but that “great numbers” of Christians “held to their determination.” So now it was best to allow them to meet freely, as long as they were orderly about it. 

Further, he declared, “It will be their duty to pray to their god for our good estate.” Rome needed all the help it could get. Galerius died six days later.
But Diocletian’s grand scheme was falling apart. When Constantius died in 306, his son Constantine was proclaimed ruler by his loyal soldiers. 

But Maximian tried to come out of retirement and rule the West again, along with his son, Maxentius (who eventually forced Maximian—his own father—out of power). Meanwhile, Galerius appointed a favorite general of his, Licinius, as the Western emperor. Each of these would-be emperors claimed a slice of the western territory. They would have to fight it out. Shrewdly, Constantine forged an alliance with Licinius and fought against Maxentius, and at the battle of Milvian Bridge, Constantine prevailed.

At that point, Constantine and Licinius forged a delicate balance of power. Constantine was eager to thank Christ for his victory, so he moved to secure freedom and status for the church. In 313, he and Licinius officially issued the Edict of Milan, granting religious freedom within the empire. “Our purpose,” it said, “is to grant both to the Christians and to all others full authority to follow whatever worship each man has desired.”

Constantine immediately took an imperial interest in the church, restoring property, granting money, ruling in the Donatist controversy, and calling church councils in Arles and Nicea. He was also jockeying for power over Licinius, whom he finally ousted in 324.

Thus the church passed from persecution to privilege. In an amazingly short time, its prospects changed completely. After centuries as a counterculture movement, the church had to learn how to deal with power. It did not do everything well. Constantine’s own dynamic presence shaped the church of the fourth century and thereafter. He was a master of power and politics; and the church learned to use those tools.

Was Constantine’s vision authentic, or was he just an opportunist, using Christianity for his own ends? 

Only God knows the soul. Though in many ways he failed to reflect his faith, the emperor certainly took an active interest in the Christianity that he professed, often at personal risk.

God certainly used Constantine to make things happen for the church; the emperor affirmed and secured official toleration of the faith. But in doing so, he followed in the footsteps of the beaten, broken Galerius who had granted it earlier. Thus the battle against Roman persecution had in a sense been won, not at the Milvian Bridge, but in the arenas, as Christians went bravely to their deaths.

Curtis, A., Lang, J. S., Petersen, R., & Curtis, J. S. L. A. K. (1998). 100 most important events in christian history, the. Grand Rapids, MI: Baker.