Why is the Holy Spirit so important? As we have seen before, He is the link, the spiritual connection between us and the Kingdom of God. He is the one who fills us with spiritual power, guides us, leads us into the knowledge of the truth, and brings to our remembrance all the things that Jesus taught. Jesus promised His disciples that after He left, He would send the Holy Spirit to be with them forever.
The Holy Spirit could not come until Jesus had completed His work on the cross, risen from the dead and applied His blood for the cleansing of man's sin. Once we were cleansed and made holy again, we were ready to receive the Holy Spirit. Jesus took care of that, too, before He ascended into heaven.
19) Then, the same day of the evening, being the first day of the week, when the doors were shut where the disciples were assembled, for fear of the Jews, Jesus came and stood in the midst and said to them, "Peace be with you". 20) When He said this, He showed them His hands and His side. Then the disciples were glad when they saw the Lord. 21) Jesus said to them again, "Peace to you! As the Father has sent Me, I also send you". 22) And when He had said this, He breathed on them, and said to them, "Receive the Holy Spirit 23) If you forgive the sins of any, they are forgiven them; if you retain the sins of any, they are retained". (John 20:19-23)
In the evening of the very day He rose from the dead, Jesus appeared to His disciples, breathed on them, and imparted the Holy Spirit to them. Later, on the day of Pentecost, they would receive the infilling of the Holy Spirit in power, but here Jesus released the Spirit to them as a continuing indwelling presence.
By this act, Jesus returned to mankind that which Adam and Eve had lost in Eden. The link was restored. All who would believe and trust in Christ for the forgiveness of their sin and commit their lives to Him would receive the Holy Spirit and thus regain their connection to citizenship in the Kingdom of God. They could then know the will of God and through them His will would be done on Earth.