Acts 2. The outpouring of the Holy Spirit on the 120 disciples gathered in Jerusalem fifty days after the Passover at which Jesus was crucified. The day was the Jewish Feast of Weeks (Shavuot), which commemorated the giving of the Law at Sinai fifty days after the first Passover in Egypt. Jerusalem was packed with pilgrims from across the Jewish diaspora. A sound like a mighty rushing wind filled the house; tongues of fire appeared and rested on each disciple; they began to speak in other languages. The Jewish crowd heard the gospel preached in their own tongues, and Peter's sermon that day saw 3,000 converted and baptized. The Church as the Spirit-indwelt body of Christ begins at Pentecost.
Pentecost is the third great pivot of redemptive history (alongside Creation and the Incarnation). Four dimensions. (1) Promise fulfilled — Joel 2:28-29 had prophesied an outpouring of the Spirit on "all flesh"; Ezekiel 36:26-27 had promised a new heart and a new Spirit. Jesus had promised "you will receive power when the Holy Spirit has come upon you" (Acts 1:8). Pentecost is the fulfillment. (2) New Covenant inaugurated at scale — the Spirit is now poured out on every believer, not merely on select prophets, priests, or kings. "In the last days, it shall be, God declares, that I will pour out my Spirit on all flesh... even on my male servants and female servants" (Acts 2:17-18). The democratization of the Spirit defines the New Covenant Church. (3) Reversal of Babel — at Babel (Genesis 11) human unity in rebellion was scattered by the confusion of languages; at Pentecost divided humanity is united by the gospel proclaimed in every language. The Spirit gathers what sin scattered. (4) Birthday of the Church — the Church existed in OT saints and in Jesus' disciples, but the Spirit-indwelt, Spirit-empowered, missionary Body of Christ begins on this day. The gospel goes from Jerusalem to Judea to Samaria to the ends of the earth, fueled by the Spirit poured out at Pentecost. Every revival in church history is a local echo of Pentecost; the original never truly ended — the age of the Spirit continues.