I have been pondering the significane of that statement for about the last hour and a half and I am finding it difficult to write. This is not because there is nothing to say but because there is too much to say. It is difficult to overstate the significance of the story of Pentecost that Luke narrates in Acts 2:1-21.
Pentecost is a reversal of the Tower of Babel. Genesis 11 tells the story of some of the earliest human beings trying build a tower that would reach to heaven, apparently a monument to humanity's own greatness. God prevents this idolatry by causing all the people to speak in different languages so that they could not communicate with one another. In contrast, at Pentecost the Holy Spirit enables the apostles to speak so that those present can all understand them in their own language. God's action at Babel divides so as to prevent idolatry. God's action at Pentecost unites those who hear to the glory of God.
Pentecost is the beginning of the harvest. Before it was a Christian festival, Pentecost was a Jewish festival also known as the Feast of Weeks. This feast was celebrated seven weeks from the time "you begin to put the sickle to the standing grain" (Deut 16). Luke's mentioning of this festival at the beginning of Acts 2 may be a pragmatic explanation as to why there were Jews from so many other parts of the world present in Jerusalem on that day since they would have gathered for the feast. However, it is also likely that their presence in the story is an indication that through the pouring out of the Spirit, the great harvest has began.
Pentecost is what was spoken of by the prophet Joel. Peter, empowered by the freshly outpoured Spirit, stands up and begins a speech by quoting Joel 2:28-32 and saying that what the crowd is observing is what Joel spoke about. This appears an odd statement at first since part of what Peter quotes is to say that there will be "blood and fire and billows of smoke. The Sun will be darkened and the moon turned into blood", none of which happens before or during Peter's speech. However, when we look at Joel's words in their context, we realize that these words are a metaphorical way of saying that one day God will pour out his Spirit on all people and when he does it will be a world changing event. It will be such a cataclysmic event that it will be like the sun being darkened or the moon being turned to blood in its world altering significance.
Pentecost signifies a new age in which Jesus is the fulfillment of God's promises. It should be obvious from the three Old Testament references above (Babel, Feast of Weeks, and Joel) that Pentecost is not something that can be understood apart from the story of Israel. Pentecost is the ultimate fulfillment of God's promises to his people. That is why Peter quotes the prophet Joel. It is a way of saying "This is what we've been waiting for!" The irony, of course, is that the promises were fulfilled in an unexpected way; through a crucified and resurrected Messiah who has now ascended into heaven rather than staying to rule over his kingdom. We do ourselves a tremendous disservice when we limit salvation to "Jesus died for my sins." Salvation is the fulfillment of Israel's hopes through Jesus and the pouring out of the Holy Spirit upon Jews first and then Gentiles alike.
God's Holy Spirit abiding among us, making us to be the presence of Jesus in his absence, destroying the barriers that divide us, leading us to uncommon people in uncommon places, overturning the process of death itself, and causing us to point away from ourselves and toward Jesus, is the agent of God's promised renewal in our world.
May God's Spirit renew us this Pentecost.