4 When the work which the Father gave the Son to do on earth was accomplished, the Holy Spirit was sent on the day of Pentecost in order that He might continually sanctify the Church, and thus, all those who believe would have access through Christ in one Spirit to the Father. He is the Spirit of Life, a fountain of water springing up to life eternal. To men, dead in sin, the Father gives life through Him, until, in Christ, He brings to life their mortal bodies. The Spirit dwells in the Church and in the hearts of the faithful, as in a temple. In them He prays on their behalf and bears witness to the fact that they are adopted sons. The Church, which the Spirit guides in way of all truth and which He unified in communion and in works of ministry, He both equips and directs with hierarchical and charismatic gifts and adorns with His fruits. By the power of the Gospel He makes the Church keep the freshness of youth. Uninterruptedly He renews it and leads it to perfect union with its Spouse. The Spirit and the Bride both say to Jesus, the Lord, "Come!" Thus, the Church has been seen as "a people made one with the unity of the Father, the Son and the Holy Spirit."
Source: Lumen Gentium (Vatican.va)