There is no exact date when Christianity began of course. But the following can serve as an overview of how it progressed and spread through the Roman Empire.
From the Pentecost to AD 313
The Bible states that the church itself commenced on Pentecost. That was when the Holy Spirit descended on the disciples (Acts 2:4). The disciples went out to spread the word of the risen Christ through the Empire. The new religion increased rapidly. The common language (Greek and Latin) made it accessible to a lot of people. It also helped that Rome was tolerant of emerging religions.
The Early Church
There were no churches when Christianity began. Instead the people met in homes and synagogues. There the learned would speak and sermon about the teachings of Christ. These gatherings would eventually grow and people began accepting the religion in droves.
The religion eventually came under Roman persecution. This happened because the Christians refused to worship the emperor as a god. Second they refused to join the military. Third their growth meant people no longer donated money to the pagan temples, which posed a burden to the treasury.
The tolerance of Rome to religions ended when it affected the empire. Although when Christianity began it was welcomed by Roman officials, by 250 it was declared illegal.
The greatest wave of persecutions came under the reigns of Valerian and Diocletian (284-305). According to historians, Christian books were burned. Christians themselves were crucified and tortured to death. Most were thrown into the Coliseum and fed to lions. There is no consensus to the number killed, but it is held to be in the thousands.
The Edict of Tolerance and the Councils
In AD 313 the emperor Constantine issued a decree of tolerance. Christians were allowed to return to the land. From tolerance the point came in 380 when it became the official religion of Rome.
From AD 325 to 387 seven councils were conducted to define its tenets and doctrines. When Christianity began, there were several gospels and the bishops decided which were genuinely inspired and were fraudulent.
Several other issues were discussed by the bishops. These meetings were presided over by the Roman emperors. From these discussions came the books that were to be included in the Bible. In 397 the New Testament canon was established and approved.
Even after the fall of Rome, the religion grew. The collapse of Rome saw the emergence of the Holy Roman Empire. By this time Europe was on the way to being Christianized. When the Europeans made their way to the New World, they carried the religion with them.
As time went by, fractions developed within the Church. Roman Catholicism remains the largest denomination followed by different forms of Protestantism. Various sects like the Jehovah’s Witnesses, 7th Day Adventists and Mormons have also spread through the world.
When Christianity began there seemed little chance that it would survive, let alone spread. But it did. Today it continues to have steady and widespread following all over the world.