Christianity is one of the three missionary religions; the others being Islam and Buddhism. All three have morphed themselves at times to meet the needs of particular groups and cultures. The success of Christianity, however was its audience. Christianity appealed to the poor and unfortunate in society. Jesus in one of his first Sermons said
The spirit of God is upon me, to teach good news to the poor.
During his ministry, Jesus often ministered to the sick and poor. It offered hope to those who had none.
A second reason was its acceptance within the Roman Empire. Interest in the old Greco-Roman gods had declined; and with its popularity among the poor, the emperor Constantine for purely practical reasons legalized the religion. Because of its popularity it soon became the official religion of the empire, and spread throughout it. It is no coincidence that after the decline of Rome, Europe was known as Christendom.