From "Lumen gentium", the Second Vatican Council's dogmatic constitution on the Church
The mission of the Holy Spirit in the church

When the Son completed the work with which the Father had entrusted him on earth, the Holy Spirit was sent on the day of Pentecost to sanctify the Church unceasingly, and thus enable believers to have access to the Father through Christ in the one Spirit. He is the Spirit of life, the fountain of water welling up to give eternal life. Through him the Father gives life to men, dead because of sin, until he raises up their mortal bodies in Christ.
The Spirit dwells in the Church and in the hearts of the faithful as in a temple. He prays in them and bears witness in them to their adoption as sons. He leads the Church into all truth and gives it unity in communion and in service. He endows it with different hierarchical and charismatic gifts, directs it by their means, and enriches it with his fruits.
By the power of the Gospel he enables the Church to grow young, perpetually renews it, and leads it to complete union with its Bridegroom. For the Spirit and the Bride say to the Lord Jesus: “Come!”
In this way the Church reveals itself as a people whose unity has its source in the unity of Father, Son and Holy Spirit.