Monotheism is simply the belief in one and only one supreme god. In ancient times this absolutely opposed the popular polytheism such as paganism, which had many hundreds and many thousands of gods. One of the most known and important monotheistic religions/ cultures was that of the Jews in Judea representing Judaism. The Birth of Christianity was conducted by and heavily influence by the Jews.
Jesus of Nazareth and his Jewish followers gave birth to Christianity. Jesus began his movement in a region of intense political and religious rivalry. It was the perfect playground for a new player or "messiah" such as Jesus. His movement only began after his mentor John the Baptist was executed by the Romans. John's death was the beginning of the Jesus movement, spread by the 12 Apostles.
The 12 Apostles were Jesus's followers during his life, and they were his preachers, his leaders of Christianity after his death. Their fervor and piety gained support and conversion gradually throughout the Roman Empire, even under heavy persecution. The 12 Apostles were Peter, Andrew, James, John, Philip, Nathanael, Mathew, Thomas, James, Simon, Jude, and Judas Iscariot.
Paul the Apostle had been a pious persecutor of the Christians. That is until his miraculous conversion after he saw Jesus (after his death). Paul is significant in that he bagan to preach and convert non-jews. Until now Jesus and the 12 Apostles had targeted only Jews and saw their movement for only the Jews. The entrance and conversion of Paul into Christianity his where most people see the split from Judaism into it's own religion. He was eventually executed by the Romans in 67 CE.
The early Christians were very divided on the essence of Christianity. What was their message? Who could be a Christian? Do they have to be Jews? These questions divided the early Christians. There was no leader of the movement. Peter who had led the 12 Apostles was executed by Rome in 64 CE. After Peter's death, a split formed on whom should inherit his authority, the Bishops of Rome or Jerusalem. For the first 3 centuries after Jesus's death, the many Christian communities all differed in their beliefs, they were not united. The only thing things that put them together was their origin, their god, and the equal persecution they faced, the persecution by Rome.
The Roman Empire, a predominantly polytheistic state heavily persecuted the early Christians. The Romans despised Christianity. First the saw it as disloyal, dis-uniting, un-roman, and utterly repulsive, especially regarding the symbolic consumption of the flesh of god. (Wine and Bread) The Romans also disliked the Christians due to their connections/origins with the Jews. The Roman persecution of the Christians killed tens of thousands. It took the form of social disdain, exile, execution, torture, rough violence, etc. It became national policy in the 1st century.
Stoicism which began in the Hellenistic Era, was adapted by the Romans into their culture and value system. Early Christians saw connections to it and Christianity and tried to exploit that and use it to gain ground/conversion in the Roman Empire. It believed in order, acceptance of divine law, discipline, and service to the community.
Neo-Platonism is a philosophical and religious system created by the followers of Plato. It favored polytheism but did have some attractions to the early Christians. Neo-Platonism created an intellectual surge in the Roman and Christian worlds. Once incorporated into Christianity, I added much needed emotional sophistication.
Constantine the Great was a emperor from 306-337. He was highly competent and led a momentary resurgence/revival of Roman stability and strength. He is most known however for his being the first Emperor to convert to Christianity. He did so upon having a dream telling him to convert so as to become victorious in the current civil war (this taking place in 306). It is believed to have been an honest conversion since there was no advantage to it. At this point Christians made up no more than 2-3 % of the population of the empire. HE immediately recognized the lack of unity within the religion and summoned all leaders to council. After 2 years that council established a united Christendom with the Nicene Creed, the universal standard of Christianity.
In 313 Constantine legalized Christianity in all the empire and set as policy the freedom of faith for all religions. Though at this point paganism was still legal and dominant, Christianity gained the momentum and within decades became dominant, and the traditional roman gods, illegal and forgotten. In 391 Christianity became the official religion of the empire.