Acts 1:4-5 (NIV) - On one occasion, while he was eating with them, he gave them this command: "Do not leave Jerusalem, but wait for the gift my Father promised, which you have heard me speak about. For John baptized with water, but in a few days you will be baptized with the Holy Spirit."
John the Baptist urged all who came to him to be baptized to prepare the way for the coming Messiah. In Jewish culture this baptism was an outward declaration of cleansing and a decision to turn back to God. By being completely immersed in water from head to toe, they were symbolically cleansed of sin and beginning anew. Their hearts would be prepared to really see and hear Him and their minds would be prepared to accept Him as the One they had hoped and prayed for for hundreds of years.
When Jesus came to be baptized He did not need to be cleansed or to turn back to God, but He set an example for all who would follow Him. As He said, "To fulfill all righteousness"; to do all that was right in God's sight.
At the final meal with His closest followers, Jesus promised to send a Counselor who would be with them always and never leave. His Spirit who would continue His work through them.
For many days after His death and resurrection He continued to give final instructions to those who would carry on the work He had begun. Before He ascended to heaven to be with His Father and prepare our place, He instructed His followers to wait in Jerusalem. In a few days, since He would no longer be with them, He would give His Spirit to live with them and guide them as He had.
This is His promise to all who follow and trust in Him. We no longer need to go and wait for His Spirit. At the moment we sincerely open our hearts and accept His gift of forgiveness and life we are immersed in His Spirit. We are cleansed and our lives begin anew. We are given His Spirit as a seal, a promise, of our eternal life with Him. His Spirit is always with us; guiding, teaching and helping.