When Jesus ascended into heaven 40 days after Easter, he instructed his Apostles to wait in Jerusalem for the sending of the Holy Spirit. Ten days later, on the Lord’s Day, the Apostles together with Mary were praying in the Upper Room when 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 Firstfruits, 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 as a result, 3,000 souls were baptized and added to the Church that day. Because of this, Pentecost is considered the birthday of the Catholic Church.