The day of Pentecost is the day of the birth of the Church, thus it is described in the gospels:
"When the day of Pentecost came, they where all together in one place. Suddenly a sound like the blowing of a violent wind came from heaven and filled the whole house where they were sitting. They saw what seemed to be tongues of fire that separated and came to rest on each of them. All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them.
Now there where staying in Jerusalem God fearing Jews from every nation under heaven. When they heard this sound, a crowd came together in bewilderment, because each one heard their own language being spoken. Utterly amazed, they asked; "Aren' t all these who are speaking Galileans? Then how is it that each of us hears them in our native language? Parthians, Medes and Elamites, residents of Mesopotamia, Judea and Cappadocia, Pontus and Asia, Phrygia and Pamphylia, Egypt and the parts of Libya near Cyrene; visitors from Rome; Cretans and Arabs - we hear them declaring the wonders of God in our own tongues!" Amazed and perplexed, they asked one another, What does this mean? Some, however made fun of them and said, "They have had too much wine."" Acts 2:1-13
The text is describing the arrival of the divine wisdom on the apostles, in philosophical terms this is what is called Gnosis. The disciples receive the "Agape", the love of charity, which translates into the 7 gifts of the Holy Spirit: gift of wisdom (love for virtue), gift of discernment, gift of counsel (listen, meditate, decides), gift of mercy, gift of knowledge (progressive thinking), gift of fear of God (perseverance, firmness, devotion, equanimity, stability) and gift of fortitude (continuous improvement).
The tongues of fire on Pentecost symbolize the purification of the souls by the wisdom of God, God grants them their essence, the wisdom, to spread the kingdom of God among men. Receiving the holy spirit means in practical terms that God makes us alike. to him, he gives us his conformity; the holy spirit transmutes the essence, keeping the substance constant. The church is born in practical terms with the spiritual adulthood of the disciples. Pentecost is an Easter, a step, of the kingdom of God as hidden wisdom, to become good news.