Following the crucifixion of Jesus, the Apostles had been very much afraid. Even though Jesus had tried to prepare them, they really did not understand what had happened to Him and they didn’t want the same thing to happen to them. So they locked themselves away and tried to avoid the attention of the authorities. But on Pentecost, the Holy Spirit descended upon the Apostles  and gave them the courage to preach the Gospel of Jesus and to baptize those who believed. On that first Pentecost Sunday, about 3,000 people were converted and baptized. These people went on to share their new faith in Christ with others. That is why this Feast is often referred to as the birthday of the Catholic Church.

When the day of Pentecost had come, they were all together in one place. 2 And suddenly from heaven there came a sound like the rush of a violent wind, and it filled the entire house where they were sitting. 3 Divided tongues, as of fire, appeared among them, and a tongue rested on each of them. 4 All of them were filled with the Holy Spirit and began to speak in other languages, as the Spirit gave them ability. (Acts 2: 1-4)

On Pentecost Sunday, when the Holy Spirit descended upon the Apostles, they received the gifts of the Holy Spirit. Those gifts helped them to fulfill their mission to preach the Gospel to all nations. Those same gifts—wisdom, understanding, right judgment, courage, knowledge, reverence and wonder and awe – also help us to live a Christian life.

Pentecost is one of the oldest of the Church’s feasts. It was celebrated early enough to be mentioned in both the Acts of the Apostles  - 16 For Paul had decided to sail past Ephesus, so that he might not have to spend time in Asia; he was eager to be in Jerusalem, if possible, on the day of Pentecost. (Acts 20:16) - and in the First Letter of Saint Paul to the Corinthians - 8 But I will stay in Ephesus until Pentecost, (1 Corinthinians16:8).

Follow this link to read what the Catechism of the Catholic Church says about Pentecost and the Holy Spirit.