September 24, 312 was the start of Imperial Indication. From the article:
"Constantine and Christianity : 312
His conversion happened during a war against his brother-in-law and co-emperor, Maxentius. According to the historian Eusebius (see Eusebius Pamphilus ), bishop of Caesarea in Palestine, before the crucial battle of Milvian Bridge, Constantine was convinced that he needed divine assistance. While he was praying for such assistance, God sent him a vision of a cross of light at midday, bearing the inscription "in hoc signo vinces " ("in this sign you will be victorious"). That night he had a dream that reaffirmed his earlier vision. God told him to use the sign he had been given as a safeguard in all of his battles. Thus, Constantine converted to Christianity and ordered the symbol of his Savior's name (the intersection of the Greek letter chi and rho) to represent his army. Constantine was victorious in the battle of the Milvian Bridge, and he continued to wear the symbol for Christ against every hostile power he faced.
Some argue as to whether or not Constantine's conversion experience was authentic. Some hypothesize that the "vision" Constantine saw was nothing more than a form of the rare natural event called the "halo phenomenon." This is caused by the sun reflecting off of ice crystals instead of rain in a rainbow. However, most historians accept Constantine's statement since he gave the testimony on oath. Also, Constantine showed sound judgment many times over in other situations. Further, Constantine did not recount this vision to Eusebius until long after that battle had been won and he was in power. Thus, he was not using the story as a tool to gain acceptance. There is no way to "prove" the event, of course, but what is important is that Constantine believed it to be true.
His conversion helped Christianity in many ways. Followers were safe from persecution, and Christian leaders were given many gifts by the Emperor. 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.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, possibly indicating that he never understood the theology of Christianity. From 320-330 he began to attack paganism through the government but in many cases persuaded 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.Whether or not his conversion was "genuine," Constantine's reign was extremely important to the Christian church. After his vision, he immediately declared Christianity legal in the Edict of Milan. He completely abandoned paganism and put his full force of favor towards advancing the cause of the Church of Christ. He provided Christianizing legislation on such matters as the observance of Sunday, the confiscation of the temple treasures, and the exemption of some clergy from taxes. He funded Christian leaders and the construction of churches, some of which he dedicated to his mother. Most Christian leaders greatly admired Constantine for the works he did for the church and Christian cause.While Constantine's idea of an integrated Church and State, (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 to settle a dispute they were having with the church in North Africa over the personhood 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 dominion over the east and the west to uproot paganism where tolerant Licinus had not. He also called together and presided over the Council of Nicaea that 300 bishops attended, which again dealt with the Arian controversy about the nature of the divinity of Jesus. The Council issued an official statement of creed affirming Jesus' complete divinity, and the decision was enforced politically by Constantine. the dispute over the personhood of Christ. They drafted the Creed of Nicaea, the predecessor to the Nicene Creed, a proclamation of faith still used by many Christian denominations today."