"A few weeks after Jesus’s crucifixion, some of his followers in Jerusalem spoke publicly to a crowd of pilgrims who had come from all parts of the Roman Empire for the feast of Pentecost. They announced that Jesus had not only risen from death but had been exalted by God to be ruler over the whole universe and that he had given them the right to offer forgiveness of sins and the gift of the indwelling Spirit of God to all who confessed him as Lord."

"An increasing number of people did so, and formed a community of believers in Jesus - mainly in Jerusalem but also in some other centers in Judea. The leaders of this new movement won popular good will by performing acts of healing such as had marked Jesus’s ministry and also by attending the temple services regularly and by keeping the Jewish religious rules. In fact, they observed them more closely than their Master had done."
(Bible History Atlas - F.F. BRUCE)