Luke seems to be the first author of the Christian Scriptures to presume both he and all who read his double volume work will die before Jesus' Second Coming. That means he's preparing his community for the long haul. Unlike his predecessors who thought they were training sprinters, Luke's coaching distance runners. Given that change in direction, there's a big question he must answer: "How do we know those who are committed to carrying on Jesus' ministry are actually doing what Jesus wants them to do?" He certainly didn't leave them any detailed instructions. Luke's answer is simple and to the point. "He left us his Spirit."
Throughout the Acts of the Apostles, the Spirit guides the early Christian community, even leading them down roads the historical Jesus had never traveled. That's why Luke spends so much time and space narrating the arrival of the Spirit just a few days after Jesus' ascension.
Pentecost was celebrated by Jews long before Jesus' birth. It was the feast which commemorated Yahweh entering into the Sinai covenant with the Chosen People. Yahweh promised to be their God and they committed themselves to being Yahweh's people. At that point the Jewish community formally came into existence.
Luke places the Spirit's arrival on this specific day because he believes it parallels what had happened 1,200 years before. This event formed Jesus' disciples into the church he intended them to be: the community which would carry on his work. All its members were now "other Christs."
It's no accident that Luke accompanies the Spirit's arrival with wind, noise and fire; elements which cause quite a disturbance. He presumes whenever the Spirit comes, someone will be disturbed. In most cases, we'll be asked to do something we've never done before.
Writing about ten years after Luke, John takes for granted the Spirit will lead the church in new directions, even beyond those the historical Jesus had explored. Listen carefully to what John's Jesus tells his disciples during the Last Supper. "I have much more to tell you, but you cannot bear it now. But when he comes, the Spirit of truth, he will guide you to all truth." In other words, "You ain't seen nothing yet!"
I've often reflected on how fortunate young couples are on the day they take their marriage vows. At that point they don't know everything those vows will one day demand of them. John's Jesus is saying the same thing about the commitment his followers are making to carry on his work. Just as the couple's love for one another will help them stay committed as the future evolves, so the Holy Spirit will help Christians stay on the right path as they evolve in their following of Jesus.
Lest we think the Spirit comes only to one group of Christians in the church, Paul reminds his Corinthian community that everyone constantly receives that Spirit. "To each individual the manifestation of the Spirit is given for some benefit." If each of us isn't open to the Spirit working in everyone, the common good of the community suffers. We're only the Body of the Risen Christ when we're attune to where the Spirit is leading that Body.
When the late Carroll Stuhhnueller was once asked by a group of women religious how they would know when the Spirit was speaking to them, he replied, "I have two rules. First, the thought you surface at that moment can't come from a process of reasoning. It has to burst into your mind like an insight. Second, if you carry out what the thought demands, it'll cost you big time." The cost is almost always the element which stops us from carrying out the Spirit's wishes. We simply don't want to be that "disturbed."