they were all in one place together.
And suddenly there came from the sky
a noise like a strong driving wind,
and it filled the entire house in which they were.
Then there appeared to them tongues as of fire,
which parted and came to rest on each one of them.
And they were all filled with the Holy Spirit
and began to speak in different tongues,
as the Spirit enabled them to proclaim.
-- Acts 2:1-11
On the day of Pentecost, the Spirit of the Promise was poured out on the disciples, gathered "together in one place." While awaiting the Spirit, "all these with one accord devoted themselves to prayer." The Spirit who teaches the Church and recalls for her everything that Jesus said was also to form her in the life of prayer.
-- CCC 2623
Fire. While water signifies birth and the fruitfulness of life given in the Holy Spirit, fire symbolizes the transforming energy of the Holy Spirit's actions. The prayer of the prophet Elijah, who "arose like fire" and whose "word burned like a torch," brought down fire from heaven on the sacrifice on Mount Carmel. This event was a "figure" of the fire of the Holy Spirit, who transforms what he touches. John the Baptist, who goes "before [the Lord] in the spirit and power of Elijah," proclaims Christ as the one who "will baptize you with the Holy Spirit and with fire." Jesus will say of the Spirit: "I came to cast fire upon the earth; and would that it were already kindled!" In the form of tongues "as of fire," the Holy Spirit rests on the disciples on the morning of Pentecost and fills them with himself The spiritual tradition has retained this symbolism of fire as one of the most expressive images of the Holy Spirit's actions. "Do not quench the Spirit."
-- CCC 696
This fullness of the Spirit was not to remain uniquely the Messiah's, but was to be communicated to the whole messianic people. On several occasions Christ promised this outpouring of the Spirit, a promise which he fulfilled first on Easter Sunday and then more strikingly at Pentecost. Filled with the Holy Spirit the apostles began to proclaim "the mighty works of God," and Peter declared this outpouring of the Spirit to be the sign of the messianic age. Those who believed in the apostolic preaching and were baptized received the gift of the Holy Spirit in their turn.
-- CCC 1287