The theology of division.

So we reach the point in Brigg’s posting of the Summa when the question of if the Holy Spirit proceeds from the Father and the Son or not occurs.

This led to division between the Orthodox and the Western Church at the end of the first millenium, because of the phrase "proceedeth from the father and the son" (Filioque in Latin). The Nicene Creed, as in the 1928 Book of common Prayer

I BELIEVE in one God the Father Almighty, Maker of heaven and earth, And of all things visible and invisible: And in one Lord Jesus Christ, the only-begotten Son of God; Begotten of his Father before all worlds, God of God, Light of Light, Very God of very God; Begotten, not made; Being of one substance with the Father; By whom all things were made: Who for us men and for our salvation came down from heaven, And was
incarnate by the Holy Ghost of the Virgin Mary, And was made man: And was crucified also for us under Pontius Pilate; He suffered and was buried: And the third day lie rose again according to the Scriptures: And ascended into heaven, And sitteth on the right hand of the Father: And he shall come again, with glory, to judge both the quick and the dead; Whose kingdom, shall have no end. And I believe in the Holy Ghost, The Lord, and Giver of Life, Who proceedeth
from the Father and the Son; Who with the Father and the Son together is worshipped and glorified; Who spake by the Prophets: And I believe one Catholic and Apostolic Church: I acknowledge one Baptism for the remission of sins: And I
look for the Resurrection of the dead: And the Life of the world to come. Amen.

The Orthodox version form the Orthodox Church in America is as follows.

We believe in one God, the Father Almighty, Maker of heaven and earth, and of all things visible and invisible. And in one Lord Jesus Christ, the Son of God, the only-begotten, begotten of the Father before all ages. Light of Light; true God of true God; begotten, not made; of one essence with the Father, by whom all things were made; who for us men and for our salvation came down from heaven, and was incarnate of the Holy Spirit and the Virgin Mary, and became man. And He was crucified for us under Pontius Pilate, and suffered, and was buried. And the third day He rose again, according to the Scriptures; and ascended into heaven, and sits at the right hand of the Father; and He shall come again with glory to judge the living and the dead; whose Kingdom shall have no end. And [we believe] in the Holy Spirit, the Lord, the Giver of Life, who proceeds from the Father; who with the Father and the Son together is worshipped and glorified; who spoke by the prophets. In one Holy, Catholic, and Apostolic Church. I acknowledge one baptism for the remission of sins. I look for the resurrection of the dead, and the life of the world to come. Amen.

The Orthodox Church notes that this is foundational.

This whole Symbol of Faith was ultimately adopted throughout the entire Church. It was put into the first person form “I believe” and used for the formal and official confession of faith made by a person (or his sponsor-godparent) at his baptism. It is also used as the formal statement of faith by a non-Orthodox Christian entering the communion of the Orthodox Church. In the same way the creed became part of the life of Orthodox Christians and an essential element of the Divine Liturgy of the Orthodox Church at which each person formally and officially accepts and renews his baptism and membership in the Church. Thus, the Symbol of Faith is the only part of the liturgy (repeated in another form just before Holy Communion) which is in the first person. All other songs and prayers of the liturgy are plural, beginning with “we”. Only the credal statement begins with “I.” This, as we shall see, is because faith is first personal, and only then corporate and communal.

So the Orthodox say that it is perilous thing to add to the creed as it was written in 328. The Western Church argued differently, and here is the justification from that Theological Ox, Aquinas.

THE HOLY SPIRIT PROCEEDS FROM THE SON

1 We find some who make this mistake about the procession of the Holy Spirit: they say the Holy Spirit does not proceed from the Son. For this reason we must show that the Holy Spirit does proceed from the Son.
2 It is manifest in sacred Scripture that the Holy Spirit is the Spirit of the Son, for Romans (8:9) says: “If any man have not the Spirit of Christ, he is none of His.” But that one might not be able to say that the Spirit that proceeds from the Father is one, and the Son’s Spirit another, it is shown from the words of the same Apostle that the Holy Spirit of the Father and of the Son is identified.
For the words just cited, “If any man have not the Spirit of Christ, he is none of His,” the Apostle added after he had said: “If so be that the Spirit of God dwell in us,” and so forth. But one cannot say that the Holy Spirit is the Spirit of Christ merely because He had Him as man, according to the words of Luke (4:1): “Jesus being full of the Holy Spirit, returned from the Jordan.” For one reads in Galatians (4:6): “Because you are sons, God has sent the Spirit of His Son into your hearts, crying: Abba (Father).”
The Holy Spirit, therefore, makes us the sons of God precisely because He is the Spirit of the Son of God. But we are made the adoptive sons of God by assimilation to the natural Son of God, as Romans (8:29) has it: “Whom He foreknew, He also predestined to be made conformable to the image of His Son, that He might be the firstborn amongst many brethren.” Thus, then, is the Holy Spirit the Spirit of Christ: so far as He is God’s natural Son. But there is no relation in accord with which the Holy Spirit can be called the Spirit of the Son of God except a relation of origin, for this is the only distinction we find in divinity. Therefore, one must say that the Holy Spirit is the Son’s Spirit by proceeding from Him.
3 The Holy Spirit, again, is sent by the Son; consider John (15:26): “When the Paraclete cometh, whom I will send you from the Father.” But whoever sends has an authority over the one sent. One must, then, say that the Son has an authority in regard to the Holy Spirit: not, of course, that of being master or being greater, but in accord with origin only. In this wise, then, the Holy Spirit is from the Son.
Now, let one say that the Son is sent by the Holy Spirit as well, because we read in Luke (4:18-21) that our Lord said Isaiah’s words (61:1) were fulfilled in Him: “The Spirit of the Lord is upon Me, He has sent Me to preach the gospel to the poor.” But consideration must be given this: the Son is sent by the Holy Spirit in accord with the assumed nature. But the Holy Spirit has not assumed a created nature, so that in accord with it He can be called sent by the Son, or so as to give the Son authority in His regard. Therefore, this remains: it is considered as an eternal person that the Son has authority over the Holy Spirit.
4 There is more. In John (16:14-15), the Son says of the Holy Spirit: “He shall glorify Me because He shall receive of Mine.” Of course, this cannot be said: He receives what is the Son’s, but does not receive from the Son; by saying, for instance, that He receives the Son’s divine essence from the Father. Hence, our Lord adds: “All things whatsoever the Father has are mine. Therefore, I said that He shall receive of Mine.” For, if all things which are the Father’s are the Son’s as well, the Father’s authority as principle of the Holy Spirit must be the Son’s as well. Therefore, just as the Holy Spirit receives what is the Father’s from the Father, so He receives what is the Son’s from the Son.
5 Here one can also introduce the testimonies of the Doctors of the Church, the Greeks included. Athanasius says: “The Holy Spirit is from the Father and the Son—not made, not created, not begotten, but proceeding.” Cyril, too, in his epistle received by the Council of Chalcedon, says: “The Spirit of the truth is named and is the Spirit of the Truth and flows from Him just as, indeed, from God the Father.” Didymus also says in his book On the Holy Spirit: “The Son is nothing else than what is given to Him by the Father, and the substance of the Holy Spirit is no other than that given Him by the Son.”
Of course, it is ridiculous that some concede that the Holy Spirit “is from the Son” or “flows from the Son” but does not “proceed from. Him.” For the verb “to proceed,” among all those which refer to origin, turns up most commonly; for, if anything is in any way at all from something, we say it proceeds from that thing. And since divinity is better designated by what is common than by what is special, in the origin of the divine persons the verb proceeding is the most suitable. And so, if one concedes that the Holy Spirit “is from the Son” or “flows from the Son,” it follows that “He proceeds from the Son.”
6 There is this, too, in the determination of the Fifth Council: “In all matters we follow the holy Fathers and Doctors of the Church: Athanasius, Hilary, Basil, Gregory the theologian, and Gregory of Nyssa, Ambrose, Augustine, Theophilus, John of Constantinople, Cyril, Leo, Proclus; and we accept what they have set down on the correct belief and the condemnation of heretics.” But it is manifest from many testimonies of Augustine, especially his On the Trinity and his Exposition of John, that the Holy Spirit is from the Son. It must, then, be conceded that the Holy Spirit is from the Son just as He is from the Father.

There are those who say that Calvin argued for a radical equality on this issue. He did not. He thought this was something beyond us, and we should stick to the basics.

Nowhere does one find Calvin giving criticisms of the traditional doctrine of the Trinity, much less can he be seen self-consciously promoting a new or unique position. If anything, Calvin is reluctant at all to speak of the Trinity. He states: If, therefore, these terms [traditional trinitarian terms] were not rashly invented, we ought to beware lest by repudiating them we be accused of overweening rashness. Indeed, I could wish they were buried, if only among all men this faith were agreed on: that Father and Son and Spirit are one God, yet the Son is not the Father, nor the Spirit the Son, but that they are differentiated by a peculiar quality.

Calvin would forego all trinitarian speculation if only men would confess the basics. He is not so naïve as to believe that this will suffice against the teachings of heretics, however, which is why he proceeds to uphold the traditional position, citing Augustine, Tertullian, Chrysostom, and Gregory of Nazianzus in the process. Calvin does, it is true, offer some criticism towards the way Hilary and Jerome use language amidst controversy, but he does not reject their theological positions. Indeed, Calvin cites the very Council of Nicaea and mentions that it represents “the agreement of the ancients.”[11] To this collection of ancients, Calvin adds his own support, and only asks that readers “impose a limit upon their curiosity” when it comes to more particular trinitarian questions. He hardly seems the trailblazer.

There are those who want to make him into a revolutionary.

I would argue instead that we should see unity among believers. Our speculation will eventually be corrected by God himself.

For he will show us all things, after he has swept the tears from our eyes. Of shame at our arrogance and hubris.