Why do Catholics call Pentecost the “birthday of the Church?”
On the feast of Pentecost, the Apostles were in the Upper Room where they held the Passover with Jesus.
All of a sudden, the Holy Spirit descended on the Apostles in the form of wind and fire. “And they were all filled with the Holy Spirit and began to speak in other tongues, as the Spirit gave them utterance” (Acts 2:4).
Then the Apostles were emboldened and rushed out to preach the Gospel and baptized 3,000 people.
We call this the “birthday” of the Church because this is the day when the Holy Spirit made the Church visible to the world.