Christianity started in the eastern Roman Empire. It gained popularity for the first time between the years of 60-65 A.D. Christianity was based on the life and teachings of Jesus Christ. It arose as a sect of Judaism (the Jewish religion), and at first it only included Jews. Christianity spread slowly throughout the Roman Empire for the next 200 years. It's rise in popularity was do to a number of factors. At the time, the world as everyone knew it was finally at peace. Basically, only one government existed, the Roman Republic. Also important, was that there was only one principle language spoken by everyone, Greek. Greek had been the primary language spoken since the times of Alexander the Great. Another important factor was that communication and travel across the vast empire was now safe and secure. Missionaries could now move freely without fear.
Christianity's central themes were readily accepted by the poorest of villages. Jesus preached love, compassion, equality, and humility. Many people, especially the poor and destitute were looking for a new religion that gave them hope. The poor heard stories of his miraculous healing, and they became his first converts. The new faith was opened to everyone who accept Jesus as their savior. Much of what Jesus preached was not new, but was instead already being practiced and believed by other established religions. Not many people converted to Christianity immediately following the Crucifixion of Jesus. Palestine was the center of Judaism, and most Jews did not believe that Jesus was their savior.
Much of the early spreading of Christianity is attributed to the missionary Paul. Paul took the message of Jesus and spread it throughout the eastern portion of the Roman Empire. He preached for decades to Hellenistic Jews and Gentiles (non-Jews)alike. In all, Paul wrote 14 books in the New Testament. Paul was executed for his teachings by the Roman Republic in 65 A.D.
At the time, Christianity was introduced the Romans had no firm commitment to any religion, and despite the fact that many Roman officials disliked Christianity, no one passed a law against it. At the time, the Romans tolerated all religions that did not threaten the empire. During this time period, mystic cults, and astrology were gaining in popularity, and Judaism was steadily finding converts.The Roman government made every attempt to promote their own religion, which required everyone to treat the emperor as a god. Daily, they offered sacrifices to the Roman gods, but there was little support for them. Everyone was required to take part in these ceremonies, but after a while many Christians refused. Before long, Christians refused to participate in the military because of Jesus teachings on pacifism.
Constantine was the first Christian emperor of the Roman Republic. He initiated a series of measures favoring Christians. In the 4th century separate Christian movements emerged. One of these called Arisus. Arisus followers did not believe in the Trinity (the Father, the Son, the Holy Ghost). In 325 A.D. Constantine called a meeting of church leader called the Council of Nicaea. The council declared Arisus heresy. Eventually, Constantine converted the entire empire to Christianity.