While the rise of Christianity to dominate western religion may very well have been inevitable, certain key moments along the way helped secure this position. The arrival of the Constantinian Dynasty was one such moment. In the early 4th century, 306 AD, Constantine rose to Emperor in the West upon the death of his father Constantius. However, he and his brother-in-law, and co-emperor in the west, Maxentius were bitter rivals. Open hostility and war broke out between the two after several years of political scheming. Before the two met in the fateful battle of the Milvian Bridge in 312 AD, Constantine supposedly had a vision of the sign of Christ in a dream. Eusebius gives an account several years later in which Christ appeared to Constantine and instructed him to place the heavenly sign on the battle standards of his army. The chi-rho symbol, or Labarum, was described by Eusebius as "a long spear, overlaid with gold", which included a bar crossing the spear to form the shape of the Christian cross. "On the top of the whole was fixed a wreath of gold and precious stones, and within this the symbol of the Savior's name, two letters indicating the name of Christ by means of the initial letters, the letter X intersection P at the center." Included with the banner were the words: In hoc signo vinces (in this sign thou shalt conquer), and armed with this holy power, Constantine crushed Maxentius securing his place as sole western Emperor.
Constantine, though previously a worshipper of Sol Invictus, the Sun God, took on support of Christianity with some zeal. He declared that his victory was owed to the god of the Christians and set about adopting an imperial policy to advance its cause. Some claims have been made that Constantine 'converted to Christianity simply for political means, and that justifiably may have played a part. Arguments have been made that Constantine was baptized years after the fact, just before his death, as a political tool to aid the accession of his sons, but it was often the custom of the early Christians to be 'cleansed' just prior to death rather than at birth. Despite these arguments, Constantine's policies and actions as emperor would indicate some considerable devotion to the Church.
Christian Bishops under Constantine functioned in an official capacity as Imperial advisors. Tax exemptions were granted to Christian priests and money was granted from the Imperial treasury to provide for new and rebuilt churches. At a meeting of Bishops in Milan (313 AD) an edict (of Milan) was passed which essentially granted complete tolerance to all religions, but Christianity would benefit the most. Previous victims of various persecutions were also granted compensation directly from the Roman treasury. Still, however, Constantine left a confusing trail for his personal religious thoughts. Association with Sol Invictus is still cited for several more years, at least until the complete unification of the Empire. The emperor in the east, Licinius maintained an adversarial relationship with Constantine for many years, which included two short wars for Imperial dominance. Licinius seems to have maintained more support for traditional pagan customs and Constantine may have resisted complete Christian conversion in order to maintain the approval of the non-Christian majority population. Perhaps in order to lure Constantine into a final battle, Licinius began inciting Pagans against Constantine's edict which favored Christianity and championed a Pagan cause. By 324 AD, the conflict and rivalry came to a head. Constantine defeated Licinius in battles at Adrianople and Chrysopolis, which ended in Licinius' capture and execution.
With Constantine's victory he became the sole ruler of the Roman Empire and likely feeling more secure in his position, began to advance the Christian cause more earnestly. New Churches were built in Rome and around the empire, such as the new basilica church on the Vatican hill, on the place where St. Peter had been martyred. The St John Lateran in Rome was commissioned and the Church of Nicomedia which had been destroyed by Diocletian was rebuilt. When the Roman capital was moved to the city of Byzantium, Constantine built new churches there as well. The Hagia Sophia (Holy Wisdom) and Hagia Eirene (Holy Peace) were built along with the foundation of the Church of the Holy Apostles. In fact, Byzantium, which was essentially a rebuilt city on old Greek ruins, was renamed Constantinople, and unlike Rome, was built with a predominately Christian flavor. His mother, Helena, after Constantine executed his own son (Crispus) and wife (Fausta) in a very un-Christian manner, embarked on a pilgrimage to the eastern provinces. There she played a part in establishing the Church of the Nativity at Bethlehem and the Church of the Eleona on Jerusalem's Mount of Olives. Perhaps more importantly, according the Eusebius she was given credit for discovering the True Cross. For this and other deeds in favor of Christianity though records seem to indicate that the True Cross had already been enshrined prior to her trip, she was canonized into Sainthood and remains recognized by both the Roman Catholic and Orthodox churches today.
Constantine also shifted to a somewhat hostile stance towards Pagans, as opposed to a simple supporter of Christianity. Pagan sacrifice was forbidden, and treasures of many temples were confiscated and given to Christian churches (excepting those temples dedicated to the Imperial cult). However, Constantine didn't direct aggression only against Pagans. 'Heretic' cults of dissension from the larger established Church cause problems as well. Among the most notable was the sect of Arianism which was deeply dividing the concept of Christian thought. At the Council of Nicaea in 325, in which some 300 bishops from all over the empire assembled to discuss the state of the church, important doctrines were developed to counter 'heretic' ideas. The core belief system of the Christian faith was developed, adopting the concept of the Holy Trinity as the supreme deity. This in itself may have included compromise between Bishops and Politicians, but it is perhaps more important that the Church was becoming a powerful and far reaching institution.
After Constantine's death in 337 AD, his son Constantine II held a tolerant, if not supportive view of the ancient Pagan faith. His second son, Constantius, was a brutal supporter of Arianism. Constans, the third son, was also a Christian, but adhered to strict Orthodoxy. A rift between sects of Christianity developed, as well as a struggle for supreme power among the brothers, causing much political instability. Constantine II was killed only a few years after his father, and the remaining brothers settled in to continue the advance of Christianity. During their reigns many anti-Pagan laws were put into place. Constans dealt with dissent in a particularly brutal fashion. The forcible expansion of Christianity on the populace, which was now quickly becoming a part of the every day social fabric, also brought a great of resentment from some. Julian the Apostate (so named later for his pro-Pagan stance) came to power upon the death of Constantius and attempted, in vain to stem the tide of Christian advancement.
Julian attempted to bring back the ancient religion to the people of the Roman world, but Christianity had become too deeply ingrained. He removed various advantages that Christian priests and churches had enjoyed since Constantine and bestowed them upon Pagans instead. Christian teachers were also removed from their occupations in many cases. Though, for the most part he avoided open violence against the Christians, he did encourage the growth of non Catholic or Orthodox sects. The fight, which could be brutal at times, for religious supremacy evolved between these various factions, but Paganism was a dying part of the dominant culture. Even temples re-established by Julian were simply overrun by fanatic Christian mobs. Despite Julian's efforts, hindered by his short reign of 2 years, Paganism continued on the path to virtual extinction.
The final death knell of the Pagan faith came only a generation later, under the rule of Theodosius. An ardent Christian, and recognizing the amazing growth of the still relatively young faith, Theodosius and his western counterpart Gratian, recognized Christianity as the official religion of the Empire in 380 AD. Gratian too, likely at the partial behest of Theodosius refused the title of Pontifex Maximus (head priest) and it was bestowed instead on the Catholic Pope in Rome. Severe punishments for Pagan, and especially 'heretic' Arianism were enforced and the established Churched prospered. In 390 AD, a massacre ordered by the Emperor of 7,000 people who revolted in Thessalonica resulted in his own 8 month penance. By the beginning of the 5th century, after just 400 years, the Church grew from a fledgling mystery cult into a power on nearly equal terms with the Roman Emperor himself. Though there would still be much work to be done, especially among Germanic tribes and in places such as Britain, Christianity would slowly come to dominate the entire western world.