When John the Baptist spoke about Jesus Christ who was coming after him, he proclaimed that he would be a man more powerful than him. John was only baptising people in water. It was a baptism of repentance. The one who was more powerful than him, Jesus the Messiah, would submerge people into union with the Spirit of Holiness and with a raging fire.
In these words John is summarising the goal of the ministry of Jesus. His ministry would be completed when he had carried away the sins of the world, broken the power of sin and death. After the resurrection Jesus showed himself alive many time to his disciples. He told them to wait for the gift the Father had promised. At that day they would be baptised in the Holy Spirit.
The coming of the Spirit on the day of Pentecost is the ultimate proof that Jesus has been glorified and exalted to the right hand of God Almighty on the throne. From now on we can all see that Jesus is the Man who is more powerful than anyone else. Even today he submerges people into union with the Holy Spirit, giving them power to be his witnesses.