Historically, the emergence of Christianity is tied to the Roman Empire that dominated the region that today is composed of Israel and Palestine. Jesus Christ was a Jew who was born in Palestine during the reign of Octavius Augustus, the first Roman emperor.
Some Jews believed that Jesus was the messiah, that is, God's envoy to redeem mankind, from which his sacred texts spoke. Others do not. Thus the Jewish authorities began to persecute him until they crucified him.
Over the next 3 centuries, Christianity was largely persecuted in the Roman Empire, until its legalization in the reign of Constantine in 313, and its subsequent formalization as a religion of the Empire by Theodosius in 390.
Jesus was followed closely by his apostles who described him as an example of principles and values and spread his teachings and stories about Jesus in Rome and in Europe. The descendants of the apostles, who began to spread Christianity throughout the world. Thus the communities constituted by the apostles were perpetuated even after their death, making Christianity strengthen as a Church.
Thus, Christianity has established itself as the greatest religion in the world, and the date of his birth is considered the initial milestone, adopted by the Christian calendar.
