It was on the Day of Pentecost that the Holy Spirit was poured out on 120 followers of Christ gathered together in Jerusalem. When this happened, great power was given to the church to fulfill the commands of Jesus and experience the promises of Jesus. Without the Holy Spirit, the church would have died before it ever started.
To this day, the church is admonished in Scripture to "be filled" with the Holy Spirit (Ephesians 5:18). How is this possible? Who is the Holy Spirit? What is His function within the Godhead? All of these questions and more are readily given to us in Scripture for our good and joy. Pentecost is not a denomination, but rather an experienced fulfillment of God's promises available to all followers of Jesus, even today. And today, just as it was then, this is only possible by the person and work of the Holy Spirit.