Pentecost is the birthday of the Catholic Church. On Pentecost Sunday we celebrate the descent of the Holy Spirit upon the apostles fifty days after the Resurrection of Christ. On Ascension Thursday, Jesus, the Second Person of the Blessed Trinity, ascended back into heaven; and ten days later on Pentecost, the Holy Spirit, the Third Person of the Blessed Trinity, descended from heaven to infuse the infant Church with life and confidence.
The Holy Spirit remains in the Church today and comes to each of us through Baptism and Confirmation.
In the words of Saint Basil:"Through the Holy Spirit we are restored to paradise, led back to the Kingdom of heaven, and adopted as children, given confidence to call God "Father" and to share in Christ's grace, called children of light and given a share in eternal glory"