The early church period is from A.D. 100 to 312 (the conversion of Constantine). The spread of the Gospel began in the eastern part of the Roman Empire and spread west. The Visigoths raided cities in Asia Minor (today Turkey)and took prisoners, many of whom were Christians. Christianity followed the Roman Empire and beyond, spreading the gospel to India and as far to Ethiopia in the south. Much of the missionary effort was done by Christians just living christian lives.
Early church leaders saw themselves as heirs to the apostles. The emphasis was on the office of bishop who was an overseer in the church. The leader of Christians in a city or region had oversight. As churches grew they needed more resources as there were more needs to meet. The problems of persecution and heresy made leadership more important.
This trend of leadership, strengthening the office of bishop, was recognized and encouraged by the church fathers. Some cities became more important and four rose to prominence: Jerusalem, Antioch, Rome, and Alexandria. Their bishops began to exercise authority as regional church governance was natural and the tendency to compete arose.
There was a controversy over when to celebrate Easter. Rome celebrated Easter on the first Sunday after the 14th of Nissan of the Jewish calendar. In Asia Minor Easter was always celebrated on the exact date that Jesus rose from the dead. The Bishop of Rome tried to command that Easter be celebrated on a Sunday. Agreement could not be reached and many church leaders objected to the arrogance of the Bishop of Rome.
Three major intellectual centers of Christianity showed the diversity of Christendom. Alexandria studied Christianity in the light of Greek philosophy. Antioch emphasized structure and order. Carthage in North Africa emphasized holiness.
The early church fathers taught the universality ("catholicity")of the faith. Like the apostles, the church fathers had a deep love for Jesus and a sense of self discipline. However, they had an unhealthy desire for maintaining order, and many emphasized works instead of grace. They tended to emphasize an allegorical interpretation of scripture.
The Apostle's Creed developed to teach new converts the elements of the faith. Remember that the Bible was not readily available. The books that formed the canon of the Bible were being gathered and collected. Before 140 we do not have a formal list.
In response to the Gnostic threat the books of the Bible that were accepted as authoritative were based on (1) Apostolic authorship (2) inherent character and worthiness and (3) general agreement among Christians.
The apologists of the early church tried to convince a hostile world that Christianity was a safe religion and there was nothing to fear. It was legitimate, safe, and true. They wrote to the emperor, pagans, and Jews that Jesus fulfilled Old testament prophecy. Christianity was rational and based on facts. Christianity contributed to the good of society.
The danger in the work of the apologists was that in trying show something was not new, they used Greek philosophy to make their arguments stronger. Pagan thought gained a bigger influence.
Gnosticism was a movement that offered secret knowledge to initiates. They said that Jesus was God, but one of many. The body of Jesus was an illusion. The Gnostic exalted the spiritual over the material and said that it didn't matter what you did with the body. Other Gnostics viewed the body as evil. The church response to the Gnostic heresies was to spell out doctrine and submit a list of books of the New Testament.