Its roots are in Judaism, for Pentecost was (and still is) a Jewish festival. Occuring 50 days after Passover and linked to both Israel's agricultural cycle and her religious history, it celebrated the completion of the spring harvest and commemorated the giving of the Law to Moses on Mt. Sinai.
According to Jewish tradition, the Law was offered to Gentile nations as well, but only Israel accepted it. Pentecost thus called to mind both the universality of Israel's God and the particularity of Israel's relationship with God.
For Christians, Pentecost celebrates the coming of the Spirit upon the followers of Jesus some 50 days after Good Friday and Easter, fulfilling a promise made by the risen Christ. The result was the beginning of the post-Easter mission of the early Christian movement.
The Christian story of Pentecost is found in the second chapter of Acts, written near the end of the first century by the same person who wrote the Gospel of Luke. The story is filled with richly symbolic language drawn from the Jewish tradition.
As the author tells the story, the Spirit came upon the community with the sound of a "rushing wind" and with "tongues of fire" resting on each of them. In the Hebrew Bible, "wind" and "fire" are both associated with the presence of God. In Hebrew, the same word means both "wind" and "spirit," as in the creation story where the divine wind (or spirit) moves over the primordial waters (Genesis 1:2).
Then, we are told, the followers of Jesus, now "filled with the Holy Spirit," began to speak in "other tongues." Often confused with "speaking in tongues" as reported in the letters of Paul, the phenomenon in Acts is not the same.
For Paul, speaking in tongues (sometimes called "glossolalia") is one of the gifts of the Spirit, and it is unintelligible speech. To quote from Paul in I Corinthians 14:2, "Those who speak in a tongue do not speak to other people, but to God; for nobody understands them, since they are speaking mysteries in the Spirit." So it is in the continuation of the practice among some Christians--called Pentecostals or charismatics--today. Glossolalia is unintelligible praise and prayer "language" addressed to God.
But in Acts, speaking in tongues at Pentecost was very different. Its effect was the opposite: Jews in Jerusalem from the many different countries and language groups of the Jewish diaspora understood, each in their own language, what the followers of Jesus were saying. They marveled: "Are not all these who are speaking Galileans? And how is it that we hear, each of us, in our own native language?" Rather than being unintelligible speech, it was supremely intelligible.
The relevant background for this part of Acts 2 is thus not the early Christian experience of glossolalia. Instead, the author of Luke-Acts is alluding to the story of the Tower of Babel in Genesis 11:1-9. This text concludes ancient Israel's symbolic narrative of human beginnings.
According to this story, the people of the earth once spoke a common language but were then scattered into different linguistic groups because of their prideful attempt to build a tower with its top in the heavens. Indeed, the English word "babble" comes from the name "Babel." Babel is the story of the fragmentation of humankind into separate and often hostile groups who do not understand each other.
Pentecost is thus about the reversal of Babel. For the author of Luke-Acts, the coming of Jesus and the continuation of his presence in the power of the Spirit inaugurated a new age in which the fragmentation of humanity was overcome. Or, in words attributed to Paul, through Christ and the Spirit, the breaking down of "the dividing wall of separation" and the creation of "one new humanity" had begun (Ephesians 2:14-15).