1 The Church professes her faith in the Holy Spirit as "the Lord, the giver of life." She professes this in the Creed which is called Nicene- Constantinopolitan from the name of the two Councils-of Nicaea (A.D. 325) and Constantinople (A.D. 381)-at which it was formulated or promulgated. It also contains the statement that the Holy Spirit "has spoken through the Prophets." These are words which the Church receives from the very source of her faith, Jesus Christ. In fact, according to the Gospel of John, the Holy Spirit is given to us with the new life, as Jesus foretells and promises on the great day of the Feast of Tabernacles: "If any one thirst let him come to me and drink. He who believeth in me as the scripture has said, 'Out of his heart shall flow rivers of living water.'" And the Evangelist explains: "This he said about the Spirit, which those who believed in him were to receive." It is the same simile of water which Jesus uses in his conversation with the Samaritan woman, when he speaks of "a spring of water welling up to eternal life," and in his conversation with Nicodemus when he speaks of the need for a new birth "of water and the Holy Spirit" in order to "enter the kingdom of God." The Church, therefore, instructed by the words of Christ, and drawing on the experience of Pentecost and her own apostolic history, has proclaimed since the earliest centuries her faith in the Holy Spirit, as the giver of life, the one in whom the inscrutable Triune God communicates himself to human beings, constituting in them the source of eternal life.
Source: Dominum et Vivificantem (Vatican.va)