Constantine was a Roman Emperor who ruled in the early 300's AD, and was arguably one of the most powerful person in his part of the world. His conversion to Christianity had far reaching effects on the common practice of the religion and on all the factions of Christianity that are present today.
His conversion happened during a war with a co-emperor, Maxentius. Constantine claimed that the night before a critical battle, in which he defeated this opponent, he had a vision of the Supreme God showing him the symbol of the cross and said, "Conquer by this." The cross was formed by the letters C (chi) and R (rho), the first two letters of the word Christ in Greek, and was the symbol that Constantine put on the shields of his soldiers. After the battle had ended, he attributed his victory to God and announced his conversion to Christianity. Eusebius, Bishop of Caesarea, recorded this account of the Constantine's conversion.
His conversion helped Christianity in many ways. Followers were safe from persecution, and the Emperor gave Christian leaders many gifts. Constantine's adherence to Christianity ensured exposure of all his subjects to the religion, and he had no small domain. He also made Sunday an official Roman holiday so that more people could attend church, and made churches tax-exempt. However, many of the same things that helped Christianity spread subtracted from its personal significance and promoted corruption and hypocrisy. Many people were attracted to the Church because of the money and favored positions available to them from Constantine rather than from piety. The growth of the Church and its new-found public aspect prompted the building of specialized places of worship where leaders were architecturally separated from the common attendees, which stood in sharp contrast to the earlier house churches which were small and informal.
Unlike modern thinking in America and other democracies, Constantine believed that the Church and the State should be as close as possible. From 312-320 Constantine was tolerant of paganism, keeping pagan gods on coins and retaining his pagan high priest title "Pontifex Maximus" in order to maintain popularity with his subjects, and possibly indicating that he never understood the theology of Christianity. From 320-330 however he began to attack paganism through the government. In many cases, Constantine was able to persuade people to follow the laws by combining pagan worship with Christianity. He made December 25th, the birthday of the pagan Unconquered Sun god, the official holiday it is now--the birthday of Jesus. It is likely that he also instituted celebrating Easter and Lent based on pagan holidays. From 330-337 Constantine stepped up his destruction of paganism, and during this time his mother, Helen, made a pilgrimage to Jerusalem and began excavations to recover artifacts in the city. This popularized the tradition of pilgrimages in Christianity.
While Constantine's idea of an integrated Church and State, a term now called Constantinism, began having sway in the empire upon his conversion, it became significantly stronger through several events. In 316, a sect of Christians called the Donatists asked Constantine, as emperor of Rome, to settle a dispute they were having with the church in North Africa over the identity of Christ. Ironically, this was resolved by Constantine favoring the N. African church. This was the first time that a political leader had power in the religious sphere.
In 324, Constantine defeated his co-emperor in the west, Licinius, leaving Constantine to rule over the east and the west. He was then able to uproot paganism where the tolerant Licinius had not. It was also during this time that he also called together and presided over the Council of Nicaea, which was attended by over 300 bishops of the Christian faith. Once again, the major topic of discussion was the identity of Christ: was He human, God, or both? They were sb;e to draft the Creed of Nicaea, which was the predecessor to the Nicene Creed, a proclamation of faith still used by many Christian denominations today. If it were not for Constantine's efforts or conversion, it is doubtful that Christianity could have survived in the harsh, persecution pagan world that existed before Constantine's reign.
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Dowley, Tim, ed. Introduction to the History of Christianity. New York: First Fortress Press, 1995.
Eliade, Mircea, ed. Encyclopedia of Religion, IV. New York: McMillan Publishing Company, 1987.