Jesus’ invitation for everyone to come and drink of the precious Holy Spirit has a condition attached. The invitation is “Whoever believes in me, as the Scripture has said…” This is an invitation, first, to believe the scriptures. Perhaps alluding to the promise in Joel 2:28-30, Jesus was inviting people to believe in Him as a prerequisite for receiving the Holy Spirit.
But His disciples, already believing in Christ, had not received the Holy Spirit up to that time. For them, this was an invitation to believe that He is interested in filling them. A case in perspective is Acts 19:1-6, where the wonderful believers at Ephesus had very impressive instruction about Christ but had no experience of the Holy Spirit. Paul clarified the importance of the experience of the Holy Spirit as accompanying (and indeed part of) the salvation experience. They believed Him.
They were rebaptized. And when Paul laid his hands on them, they were filled with the Holy Spirit.
We unblock the way for the Rivers of the Holy Spirit to flow when we exercise faith in His mighty visitation. Do you really believe that God is interested in filling you? He invites you, “be filled with the Spirit” (Eph 5:18).