The first Christian Pentecost happened 50 days after Jesus rose from the dead (see: Acts 2), when on the Jewish Pentecost (see: Leviticus 23:15-16), all believers gathered in one place, suddenly there came a sound from heaven like the rush of a violent wind, and tongues of fire appeared among them and rested on each of them. And all of them were filled with the Holy Spirit and began to speak in other languages, as the Spirit gave them ability. It is believed that the coming of the Holy Spirit is fulfilling the promise of Jesus just before the Ascension of Jesus (see: Acts 1; Luk 24).
One of the most important messages when the Holy Spirit comes upon us is there will be a spiritual growth. Paul speaks a Christian maturity, that is when a person has a wisdom of God in his/her life (1 Cor 2:6; Heb 6:1). The maturity means the lack of fleshly people, no more as an infant in Christ who need milk and cannot take solid food, whereas they are called to be animated by the Spirit, to be "spiritual", to live out their baptismal consecration, to be sanctified, to be holy, to grow above all in love of God and neighbours, to be more care for others, to be a living witness of faith.
