The Apostle explains they must repent and be baptized in the name of Jesus Christ. Then he promises more than atonement; he promises the Holy Spirit. It is one thing to get back on track with God; but being filled with the Holy Spirit is to enter a new dimension. It is more than a change from still photos to motion pictures, or from black-and-white to color; it is as radical as the shift from two dimensions to three. Normal will never be the same.
Preachers, theologians, catechists and parents have struggled throughout the centuries to say something definitive about the Holy Spirit. No sooner had the Fathers of the Church developed a clearer understanding of God the Father and God the Son than they must name God the Spirit as the third person of the Trinity. The doctrine sometimes feel like an afterthought but it is impelled by the testimony of scripture, the liturgy and everyday experience.
Saint Peter promised the crowds, "...you will receive the Holy Spirit." He could not repress that announcement. Just as it is impossible to keep joy to oneself, so must we share the Holy Spirit with others -- despite the fact we have no authority over the Spirit. Only God can bestow that gift to those who dispose themselves to receive it, to those who repent and are baptized.
It is easy to speak of the Holy Spirit but sometimes we take for granted that others know what we're talking about. I heard a sermon once on "justification by faith" and, for the life of me, I could not figure out what the preacher was talking about. It seemed to have no connection to my experience of relationships, health or happiness. He didn't even mention the Original Sin of being white, American and male. Christians use other words with that same careless abandon: saved, grace, freedom, to name a few.
I think of the Holy Spirit in terms of that freedom which God has promised to his elect. In John 8, we hear the Jews quarrel with Jesus,
"We are descendants of Abraham and have never been enslaved to anyone. How can you say, ‘You will become free’?”
Jesus insists they are not free:
“Amen, amen, I say to you, everyone who commits sin is a slave of sin. A slave does not remain in a household forever, but a son always remains. So if a son frees you, then you will truly be free. I know that you are descendants of Abraham. But you are trying to kill me, because my word has no room among you.
No one wants to be told he is not free; we like to live in the illusion that we are happy, generous, open-minded and free. But when we meet someone who is truly happy, generous, open-minded and free we realize, "This person is blessed. Can I be so blessed?"
The crowds in Jerusalem on that Pentecost saw the disciples streaming out of the Upper Room filled with the Holy Spirit and realized they were living their lives in black-and-white while these Christians enjoyed color.
Those who practice freedom in the Holy Spirit have a different normal. Jesus explained to Nicodemus, "The wind blows where it wills, and you can hear the sound it makes, but you do not know where it comes from or where it goes; so it is with everyone who is born of the Spirit.”
This will always make perfect sense to the Children of God, and will always sound like nonsense to everyone else.