When Jesus was with His disciples at the very end of His time on Earth, He commanded them not to leave Jerusalem but to wait for the promise of the Father. That promise was that in a matter of days they would be baptized with the Holy Spirit. The disciples asked Jesus if He was going to restore the kingdom on Earth to Israel. Jesus responded to them by saying it was not for them to know the times or the seasons of the Father’s plans for the kingdom. The main thing for them to know was that they would receive power when the Holy Spirit came upon them. They would be witnesses for Jesus in Jerusalem, Judea, Samaria, and to the end of the Earth.
This power they would receive would enable them to do mighty works in the name of Jesus. They would be courageous in the face of persecution. They would be given the words to speak to those who would try them for blasphemy. They would start the spread of the Gospel all over the world.
Today Christianity is the largest religion in the world. This has come about by the power of God the Holy Spirit using Christians to evangelize the world for Jesus Christ.