When Jesus ascended into heaven forty days after His resurrection, He instructed the Apostles to wait in Jerusalem for the sending of the Holy Spirit. Ten days later the eleven Apostles, together with the Blessed Virgin Mary, were praying in the Upper Room on the Lord's Day. The Holy Spirit descended upon them as tongues of fire, as recorded in the second chapter of Acts. Jews from distant lands were gathered in Jerusalem for the feast of the harvest of the first-fruits, which was the closing festival of the Pascal season. The Apostles, filled with the Holy Spirit, began to preach the Gospel of Jesus Christ in the various languages of the people. Scripture records that through this miracle 3,000 souls were baptized and added to the Church that same day. Because of this, Pentecost is considered the birthday of the Catholic Church. For more information please see morningoffering.com/2021-05-23/.