Jesus rose from the Dead and appeared to his followers. The Book of Acts states this number was about 120 members. After teaching them for a period of time (40 days), He ascended back to heaven as a "cloud took him out of their sight." The followers then returned to Jerusalem and went to the "Upper Room" where they were staying.
The leaders at this time were Peter, John, James, Andrew, Philip, Thomas, Bartholomew, and Matthew, James (Son of Aplaheus), Simon, and Judas (son of James). They were joined by many female followers of Christ as well, including Mary, the mother of Christ.
Peter immediately took the reigns he had been given as the "Rock" upon which Jesus would build his Church on Earth. He gathered them together and they discussed who would take Judas Iscariot's position as an apostle. Not able to decide, they let fate intervene by throwing lots, and the follower, Matthias, was added to the eleven apostles.
The the Day of Pentecost came and all of them felt the presence of the Holy Spirit as it came down from heaven like a "rush of violent wind" and tongues of something like fire rested on each of them as they were filled by the Holy Spirit. They began speaking in other tongues, and many foreigners heard them speaking and were shocked at this development. It caused such a commotion, that some men accused them of all being drunk "with new wine." Peter began to preach to them and told the story of Christ and all that had been foretold about him. His words and the Spirit filled the crowd and many asked what they should do.
His answer, "Repent, and be baptized." Amazingly, the Book of Acts reports that over three thousand people were saved that day and became devoted to following the apostle's teachings.
Many began to sell everything they owned to live in a commune style to learn and praise, but also looking to the day that God would return. They shared everything and "day by day the Lord added to their number those who were being saved."
Daily the apostles went to the temple and taught the Jews the good news. This disturbed the Sadducees who had Peter and John arrested for it, but they couldn't prevent the rapid growth of the Christian sect of Judaism. The Book of Acts records that the number of Christians were now over 5,000.
The leaders of the Jews brought them to trial, but they knew that Peter and John had caused a lame man to be healed and everyone was talking about the miracle. They decided to allow Peter and John to go free if they agreed to no longer do things in the "name" of Jesus. Peter, who had once denied Christ three times, refused, and in his righteousness, the leaders could not agree on a punishment and let them go.
It was only the beginning.