Christianity emerged as a leading religion in the Imperial Roman age for a variety of factors. The teachings of Christ and Christian ideology including the concept of equality in the afterlife were obvious draws. However, people gravitated towards anything that would offer a new hope, especially as the stability of the late Empire continued to unravel. Some have suggested that the spread of Christianity had direct responsibility for the fall of the Empire, but it was more a symptom of the failings of Roman culture than the cause of the fall. Rome had suffered social disorder from its very foundations. Beginning with resistance to the Etruscan Kings, the political battles between Patricians and Plebes and continuing into the social wars fought by disenfranchised Italians seeking Roman citizenship; religious change was just another result of these various social occurences. Continuing from the early Republic and the great influx of foreign slaves to the very end of the Empire with massive migratins of foreign residents, along with all their differing ideas and cultures, its not at all surprising that religion would be altered just as much as the social climate. The single God concept (monotheism) was nothing new to Romans either, though Christianity did initiate a change in philosophy where that God stood above both the Emperor and Rome itself.
Christianity had many similarities to other cults that had already gained widespread acceptance. Mithraism, derived from eastern Zoroastrism was a belief in the son of the sun who also came to earth to rescue mankind from itself. The similarities in the stories of Jesus and Mithras cannot be overlooked as an aid in Christian growth. Mithras was extremely popular in the Legions, and as the army traveled throughout the empire, the acceptance of the monotheistic concept (and the story of the son of god coming to earth to save humanity) traveled with it. The cult of Dionysus, one of the old gods of both Greeks and Romans, also had enough similarities to aid a slow conversion to Christianity. Perhaps even the Imperial cult (emperor worship) played its own part. Augustus himself was considered the son of a god (Julius Caesar) and transcended his human existence to become a divine being after his death. The Roman people had certainly been exposed to enough religious ideas bearing similarities to Christ to make the possibility of the Son of God and Savior of humanity a believable and relatively easy concept to adopt.
The idea was not so pronounced in the early empire and the foundation of the faith, however. Evidence of early Christian behavior and practices is limited, but its known that Christians weren't always of like mind and beliefs either. Several various sects with widely divergent schools of thought developed as the concept of Christ spread. Though most of the pronounced deviations from the Catholic norm, (ie. Donatism, Montanism, Gnosticism, Arianism, Pelagianism, among many others) were fairly late developments, it is evidence of widely varying views and practices regarding Christianity throughout its rise. Eventually, the Orthodox Church would gain supreme hold of the eastern empire, while Catholicism would reign in the west. The Catholic Church brought uniformity to the faith and established it as a public institution rather than small communities of individual followers. The Church not only established strict laws and religious doctrine but it wiped out 'heretic' and divergent thoughts. Sometimes through violence as severe as the persecutions against the early Christians and other times through subtle adoption of pre-existing religious concepts, the Catholic church virtually destroyed these other sects and Paganism along with it.
Early Christians, facing scorn at best and persecution at worst, depending on Emperor and the era, were forced to blend in with their Pagan counterparts. In order to celebrate the 'holidays' of their religion, the Christians used pre-existing holidays and festivals to blend in. Christmas, for example, was originally part of the great festival of the Winter Solstice, or the Saturnalia. By adopting this grand event as the celebration of Christ's birth, Christian revelry was allowed to take place, largely unnoticed. The Church too manipulated customs and traditions of the Pagan Empire to make their faith more adaptable. One of the more difficult challenges was simply getting people to believe in a single god, and give up all the others that they were accustomed to. In overcoming this obstacle, the Church began to adopt Patron Saints of various daily life functions, to allow an easier conversion. Though these Saints weren't gods in the Pagan sense, having multiple choices for the population to look to for guidance helped ease the transition. The idea of the holy trinity too, harkens to a time where people needed separate entities to spread their prayer. Even the office of the Pope as the head of the faith began to replace the Emperor in the eyes of the people as the living incarnation of God on earth.
The Church too, as it began to become an institution of considerable power in the later 3rd century, used tactics as brutal as anti-Christian Emperors. While the teachings of Christ taught love and compassion for humanity the Church itself was run by men. Like any other institution, some of these men were as motivated and power hungry as any political official in the history of Rome. Others turned a blind eye to apparent hypocritical behavior in order to advance the Church. Unlike the many cultures and beliefs of the Pagans before it, this new power was unified under a mostly single set of rules and concepts. There was one God, one set of rules and generally speaking, one way to practice the faith. By the time Christianity took firm hold on a large part of the population, people who followed Christ knew these rules and customs without the interference of other gods and their unique traditions. The Pagans with so many different ideas and traditions were unable to put up any sort of unified resistance to the juggernaut that the Church became.
By the fall of the western Empire (476 AD), Christianity was not only the official religion of the Roman world, but it had supreme authority in matters of morality and human behavior. Censorship played a large role as well. Historical documents of an incalculable number were destroyed or edited in order to prevent anti-Christian, or perceived anti-Christian thought. It is hard to imagine how much written history, and evidence of the ancient world, was lost forever due to this manipulation, but on the contrary, humanity must also recognize the great contribution of the Church to historical preservation. As the empire, the law and order of Rome and the Legions fell, there was little left to preserve the vast recorded history of the Greeks and Romans. The world shifted into a turbulent dark age where political and social instability was only countered by the constant nature of the church. Without the church, despite the historical evidence that may have been lost at the hands of fearful priests, perhaps little of the ancient world's recorded history would be available to modern man. Its stability and consistency, in the turmoil of post Roman Empire Europe, offered a perfect escape for the minds and hearts of humanity. The Church remained a beacon of hope to the greater part of the population, and it remained the last bastion of the old Roman world in a Europe that was facing massive upheaval.