We read in the Acts of the Apostles that the disciples were gathered together in prayer in the Cenacle when the Holy Spirit descended upon them with power like wind and fire. They then began to proclaim the glad tidings of Christ's resurrection in many languages (cf. Acts 2:1-4). That was the "baptism in the Holy Spirit," which had already been announced by John the Baptist: "I have baptized you with water," he said to the crowds, "but he who comes after me is more powerful than me. (...) He will baptize you in the Holy Spirit" (Matthew 3:11).
In effect, Jesus' whole mission was aimed at giving the Spirit of God to men and baptizing them in the "bath" of regeneration. This was realized through his glorification (cf. John 7:39), that is, through his death and resurrection: Then the Spirit of God was poured out in a super-abundant way, like a waterfall able to purify every heart, to extinguish the flames of evil and ignite the fire of divine love in the world.
The Holy Spirit Descends on the Disciples - by Gustave Dore
The Acts of the Apostles present Pentecost as a fulfillment of such a promise and therefore as the crowning moment of Jesus' whole mission. After his resurrection, he himself ordered his disciples to stay in Jerusalem, because, he said, "In a short time you will be baptized in the Holy Spirit" (Acts 1:8); and he added: "You will have the power of the Holy Spirit, who will descend upon you and you will be my witnesses in Jerusalem, in all of Galilee and Samaria unto the ends of the earth" (Acts 1:8).
Pentecost is, thus, in a special way, the baptism of the Church who undertakes her universal mission beginning from the streets of Jerusalem with prodigious preaching in the different languages of humanity. In this baptism of the Holy Spirit the personal and communal dimensions -- the "I" of the disciple and the "we" of the Church -- are inseparable. The Spirit consecrates the person and at the same time makes him a living member of the mystical body of Christ, participant in the mission to witness to his love.
And this is actualized through the sacraments of Christian initiation: baptism and confirmation. In my message for World Youth Day 2008, I invited young people to rediscover the presence of the Holy Spirit in their lives and, therefore, the importance of these sacraments. Today I would like to extend this invitation to everyone: Let us rediscover, dear brothers and sisters, the beauty of being baptized in the Holy Spirit; let us be aware again of our baptism and of our confirmation, sources of grace that are always present.
Let us ask the Virgin Mary to obtain a renewed Pentecost for the Church again today, a Pentecost that will spread in everyone the joy of living and witnessing to the Gospel.