Claim
The Western side: by the ninth century this clause was in common use in the West, and, as Olson puts it, widely accepted as orthodox doctrine. ⚠ And in 809 a synod at Aachen in Germany went further: it affirmed the filioque as orthodox doctrine (Pope Leo III refused to write it into the text of the creed). ⭐ A restraint worth recording: Olson notes that at the time even the pope opposed such an extreme measure, and proposed using the Apostles' Creed (the Old Roman Creed) instead, to avoid an open break with the East — but the proposal, he adds, came too late.
⚠ The West did not have only a fait accompli; it also had reasons, and we record them here. Western Trinitarian theology was rooted deeply in Augustine. Olson explains that it tended to begin from the unity of God's essence and only then to interpret the three persons of God, and it regarded the Holy Spirit as the bond of love between the Father and the Son. Because the Spirit has a uniting role in the Western scheme, Augustine in On the Trinity could say that the Spirit comes from the Father and the Son. In Olson's account: in the living God the Spirit is the principle of unity; the Spirit goes out from the Father to the Son and from the Son to the Father; so it was natural for them to say that the Father and the Son together send the Spirit into the world.
⭐ And on the side of authority, the West's reasoning was a mirror image. The East accused the West of acting without consent; the West accused the East of being unwilling to recognize the West's right to develop its own tradition. ⚠ That is, the West did not think it had changed the ecumenical creed. It thought that, within its own tradition, it had stated more clearly something that was true all along.
Counterclaim
The Eastern side had two objections, and Olson sets them out clearly. ⭐ The first belongs to this thread: they protested loudly that the West had no right to change the basic creed of Christendom without first gaining the East's consent. The second belongs to the Trinity thread: this version differed greatly from the Eastern understanding of the Trinity. To Eastern ears it implied a subordination of the Spirit and seemed to rob the Spirit of his personhood. ⚠ Note that there was something the East could say: they all agreed that it was theologically acceptable to say the Spirit proceeds from the Father, through the Son — the difference was one preposition. Meyendorff sums up the Byzantine judgment: by accepting the creed with the inserted words, the Latin church not only went against the text that the general councils had adopted as the expression of the faith of all Christians, but also gave the authority of dogma to a wrong view of the Trinity.
⚠ The Eastern Trinitarian reasons are given in full in our source text, and we record them here. The Eastern tradition, Olson explains, stresses the supreme sovereignty of God the Father: the Father is the eternal source, principle and cause of the Son and the Spirit. In this scheme, to say that the Spirit comes from the Son implies that the Son, just like the Father, is a source, principle and cause of the Spirit. That lowers the supreme sovereignty of the Father, and suggests that the Spirit is a kind of son of God the Son — even putting him on the same footing as the Son of God, who is the Son of the Father. ⭐ In the East's eyes the consequence was very grave: from this view, Olson writes, there could follow an order of the Trinity entirely different from the tradition; Eastern Christians held that it insults both the Father and the Spirit, and confuses the Son with the Father.
⚠ Please read this paragraph together with the Western one. The West starts from the unity of God's essence, and the Spirit is the bond of love between the Father and the Son, so the filioque comes naturally. The East starts from the Father as the one source, and the filioque amounts to giving the Spirit two sources. ⭐ This is not one side reasoning while the other side bullies. These are two schemes, each consistent in itself, and each followed its own logic to a place the other side could not accept.