Christianity began as a reform movement in first century Palestine which aimed to revise the teachings of Judaism and spread the teachings of Jesus of Nazareth. The early groups did not refer to themselves as Christians but the name was given to them by people from outside of the faith. The first use of the term is seen in the bible in Acts 17:26 whilst the followers were in Antioch, previous to that they had been known as Disciples, Saints or followers of the way..
The early Christian movement was quite diverse and not very well organised because there was no overall hierarchical structure and had no formal set of beliefs. Organisational structures and formal teachings and creeds began to emerge much later. The early followers were inspired by their personal experience of Jesus or from the eye witness accounts of him. After the death of the last apostle the followers chose leaders who were known as 'Fathers of the Church’ such as Athanasius of Alexandra or Augustine of hippo.
Later the teachings were formalized through Church councils and the formation of the New Testament and the Canon of scripture that we know today were agreed. The new Christian faith was rooted in Judaism and the Scriptures of the Old Testament. It worshiped the same God, The God of Abraham Isaac and Jacob but that the Messiah which means anointed one had arrived in the person of Jesus Christ whilst the Jews believed the Messiah was still to come sometime in the future. The name of Jesus is unusual in that it declares who he is. Judaism placed emphasis on the laws and commands of the prophets whilst the New Christians placed emphasis on Love, Grace and the spirit rather than the letter of the law.
Jesus had emphasized that he did not come to abolish the Jewish law but to fulfill it. Christians and Jews both agree that the scriptures carry authority from God and the early followers as did Jesus often referred to the old testament. The Jews believe that the prophesies of the old testament are complete whilst Christians believe the text is pointing forward to the future. At various points in the Gospel, events in the life of Jesus fulfill old testament prophesy.
In the early Church the identity of the new believers was recast so that circumcision and being Jewish was no longer required because the new believers held their identity in Christ and not the cultural and ethnic norms of Judaism, although for many years there was tension between believers over these issues. These problems began to dissipate as Christianity spread rapidly throughout the Greek and Roman world with new believers having no cultural ties or knowledge of Jewish culture or the Old testament. Through the missionary journeys of Paul Christianity spread widely in a Greek speaking context so rather than preaching Jesus as the fulfillment of the hopes of Israel he presented Jesus as somebody who could fulfill the deep longings of the human heart and the intuitions of human reason. This was in keeping with Greek traditions in philosophy and helped the teachings to be more universal in their appeal.
One feature of the early spread was that it was through preaching and teaching rather than by force. Force though was used against it by the imperial authorities with Christians often being persecuted. Christianity was not recognized as a legal movement in the Roman empire until the fourth century and so Christians had to meet in secret, gathering in households and yet despite this it continued to grow through a series of networks. Public proclamation would have been severely punished. At this time there was no centralized authority or Church. So why did people risk punishment and persecution? Christianity gave people a new sense of status and identity particularly for those who were poor and powerless which included women and slaves. There was practical help for those who had nothing, care for the poor, sick and needy, particularly widows. In this way the new Christians attained self-esteem and value including the idea that all people were equal and that Justice would prevail over evil.
The new faith obviously had a positive impact on their lives both spiritually and physically with the taking of Bread and Wine being described at Antioch as "The medicine of immortality" This theme of resurrection and immortality also attracted many, particularly as they noted a remarkable absence of fear in the martyrs and those who were brutally persecuted. Christianity also offered a wise moral code in contrast to the decadence of the imperial cult and the practices of pagan idol worship. But central to this growth in faith, was awe and wonder at the person of Jesus Christ himself.
Summary and References: Christian History-Alister Mc Grath.