Acts 1-2

Immediately before Jesus was taken up into heaven, He told His disciples to stay in Jerusalem until they were baptized of the Holy Spirit, which took place about 10 days later.  Jesus told them that the purpose of that baptism was to give them power, so that they might be effect witnesses for Him throughout the whole world.  The Apostles watched Jesus being taken up into the clouds, and the angel told them that it would be in a like manner that He will return, where people will see Him coming from the sky.  While they were waiting for the Holy Spirit’s baptism in Jerusalem, the apostles chose Matthias to be the 12th apostle, in place of Judas, who had given up his office in betraying Jesus.

On the day of Pentecost, which was a feast of the Jews which takes place 50 days following the Feast of First Fruits (when Jesus was raised from the dead), the Holy Spirit fell in power upon the disciples who were gathered together in Jerusalem, just as the Lord has promised.  Under the inspiration of the Holy Spirit, they began to speak in languages they didn’t otherwise know, and foreigners in Jerusalem for Pentecost heard their own native languages being spoken, and many believed after Peter stood up and preached about Jesus, and all that they were witnessing.  On that day some 3,000 people were saved, and it really marked the birth date of Christ’s church.