The book of Acts records that the disciples were together in one place when a sound like a rushing wind filled the house and tongues as of fire rested on each of them. They were filled with the Holy Spirit and began to speak in other languages, so that the pilgrims gathered in Jerusalem from across the known world each heard the gospel in their own tongue.
Peter, who weeks earlier had denied Christ, stood and preached. He proclaimed the crucified and risen Jesus as Lord and Messiah, and called the crowd to repentance and baptism. About three thousand were added that day.
Pentecost is the hinge between the gospel and the Church. What Christ accomplished, the Spirit now applied; what was promised, the Spirit now poured out. Every later chapter of Christian history — every council, every creed, every mission — begins from that upper room.