Before his Passover, Jesus announced the sending of “another Paraclete” (Advocate), the Holy Spirit. At work since creation, having previously “spoken through the prophets”, the Spirit will now be with and in the disciples, to teach them and guide them “into all the truth”. The Holy Spirit is thus revealed as another divine person with Jesus and the Father.
The eternal origin of the Holy Spirit is revealed in his mission in time. the Spirit is sent to the apostles and to the Church both by the Father in the name of the Son, and by the Son in person, once he had returned to the Father. The sending of the person of the Spirit after Jesus’ glorification reveals in its fullness the mystery of the Holy Trinity.
The apostolic faith concerning the Spirit was confessed by the second ecumenical council at Constantinople (381): “We believe in the Holy Spirit, the Lord and giver of life, who proceeds from the Father.” By this confession, the Church recognizes the Father as “the source and origin of the whole divinity”. But the eternal origin of the Spirit is not unconnected with the Son’s origin: “The Holy Spirit, the third person of the Trinity, is God, one and equal with the Father and the Son, of the same substance and also of the same nature. . . Yet he is not called the Spirit of the Father alone,. . . but the Spirit of both the Father and the Son.” The Creed of the Church from the Council of Constantinople confesses: “With the Father and the Son, he is worshipped and glorified.”
The Latin tradition of the Creed confesses that the Spirit “proceeds from the Father and the Son (filioque)”. the Council of Florence in 1438 explains: “The Holy Spirit is eternally from Father and Son; He has his nature and subsistence at once (simul) from the Father and the Son. He proceeds eternally from both as from one principle and through one spiration… And, since the Father has through generation given to the only-begotten Son everything that belongs to the Father, except being Father, the Son has also eternally from the Father, from whom he is eternally born, that the Holy Spirit proceeds from the Son.”
The affirmation of the filioque does not appear in the Creed confessed in 381 at Constantinople. But Pope St. Leo I, following an ancient Latin and Alexandrian tradition, had already confessed it dogmatically in 447, even before Rome, in 451 at the Council of Chalcedon, came to recognize and receive the Symbol of 381. the use of this formula in the Creed was gradually admitted into the Latin liturgy (between the eighth and eleventh centuries). The introduction of the filioque into the Niceno-Constantinopolitan Creed by the Latin liturgy constitutes moreover, even today, a point of disagreement with the Orthodox Churches.
At the outset the Eastern tradition expresses the Father’s character as first origin of the Spirit. By confessing the Spirit as he “who proceeds from the Father”, it affirms that he comes from the Father through the Son. The Western tradition expresses first the consubstantial communion between Father and Son, by saying that the Spirit proceeds from the Father and the Son (filioque). It says this, “legitimately and with good reason”, for the eternal order of the divine persons in their consubstantial communion implies that the Father, as “the principle without principle”, is the first origin of the Spirit, but also that as Father of the only Son, he is, with the Son, the single principle from which the Holy Spirit proceeds. This legitimate complementarity, provided it does not become rigid, does not affect the identity of faith in the reality of the same mystery confessed.
From the Catechism of the Catholic Church 243-248