(Taken from Notes Made at the Conferences of Dom Prosper Guéranger).
Qui cum Patre et Filio simul adoratur et conglorificatur. [Who with the Father and the Son together is worshiped and glorified.]
The Holy Ghost must needs be adored, therefore He is truly God. So to be in the True Faith, it suffices not to pay honour only, to the Holy Ghost, He must be adored as God, just as we adore the Father and the Son, simul adoratur: He is adored like the other two Divine Persons, and at the same time as They, simul. At these words, Holy Church wishes an inclination of the head to be made, as a homage paid to the Holy Ghost, whose Divinity we are here confessing. Et conglorificatur, He is conglorified, that is to say, He receives glory together with the Father and the Son; He is included in the same doxology, or glorification, for doxology means to give glory.
Qui locutus est per prophetas; here we have another Dogma. The Holy Ghost spoke by the Prophets, and the Church proclaims that He did so. In formulating this article she had chiefly in view the confounding of the Marcionites, who taught that there was a Good God and an Evil God; and, according to them, the God of the Jews was not good. The Church here declaring that the Holy Ghost spoke by the Prophets, from the Books of Moses, right up to those which near the time of Our Lord, - proclaims that the Action of the Divine Spirit was spread over our earth, from the very commencement. On Pentecost-Day, He came down upon the Apostles, and descended upon earth, in order to abide there: His mission being wholly different from that of Our Lord. The Word made flesh came down to our earth, but after a certain time, He ascended again to heaven. The Holy Ghost, on the contrary, came, that He might abide with us for ever: so that, our Lord, when announcing the coming Paraclete to His Apostles, said to them: The Father will give you another Paraclete, that he may abide with you for ever (St. John, xiv. 16). He added, that this Paraclete would teach them all things, by giving them the remembrance of all the things which He Himself had taught them: He will bring all things to your mind, whatsoever I shall have said unto you (Id. ibid. 26). The Church requires to be taught, guided, led, and supported. To whom does it belong to effect all this? Who is it that does it? It is the Holy Ghost, Who is to assist her even to the end of the world, according to the promise of our Lord. Thus, the Son has been sent by the Father; and then He ascended into heaven again: both the Father and the Son sent the Holy Ghost, that He might remain with the Church to the end of the world. Our Lord said: My Father will send you the Spirit: and he also said: I will send you the Spirit: and this, in order to show the relations which exist between the Divine Persons, who can never be isolated one from the other, as the heretics asserted.
Holy Church, then, has put clearly before us the dogma of the Trinity. First of all, we have the Father Almighty, Creator of all things; then, the Son, who came down from heaven, was made man, and died for us; after which He rose again from death, and ascended triumphantly into heaven, by His Ascension; finally, we have the Holy Ghost, Lord equally with the Father and the Son, the Giver of Life, who spoke by the Prophets, and is God together with the Father and the Son.