If some Hierarch in the Orthodox Church were to use a formula of Gregory of Cyprus II that the "Holy Spirit the Lord the Giver of Life Who procheisthai (flows forth) from the Father and the Son" as an interpolation in the Creed it would be "another" Creed because it goes against the intentions of the Nicene-Constantinopolitan creed even though an Orthodox expression. It would be reflecting something else than what the original text established and breaking the bond of love in the Church as she professes Her faith. Of course, I think you already know this, but it is on that basis that the "filioque" should be dropped so we can profess the original intention of our Fathers as Pope Leo III engraved on those two silver shields. I know you don't have any control of such things, but one can only hope.
This is where the difference between the usual Catholic and Orthodox perspectives on development of doctrine makes itself so keenly felt. Photios and I agree that Gregory of Cyprus’s phrase is, in its context, a good bit of triadology, in the sense that it accurately expresses an aspect of the faith-once-delivered. But I simply cannot agree with Photios that interpolating said phrase into the Creed of 381 would run counter to “the intentions” of that Symbol. If that Symbol can be said to have an intention, the intention was to express the faith-once-delivered for the universal Church, in response to some heresies of the fourth century. Now for all I know, Gregory’s expression might run counter to the pneumatology of some of the bishops active at Constantinople I; that is an intensely academic matter of opinion; but since Photios concedes that the expression is Orthodox, he is logically committed to holding that the interpolation would not go against the Faith expressed by the Creed of 381. Therefore, on my account, it cannot go against the intention of that Symbol.
Of course Photios might be urging something more specific: that “the intention” of the Fathers of Constantinople was to rule out any formal amplification of their Symbol in future, even if such an amplification would accurately express the faith of the Church. If that is so, then any formal amplification would be “reflecting something else than what the original text established”; for the text only “established” what it actually and formally says; and hence adding any truth, even by way of explication and amplification, would be adding “something else” just in virtue of the formal difference. But why would interpolating an agreed-upon truth, by going beyond what the text formally says, thereby constitute “breaking the bond of love in the Church as she professes Her faith”? Ex hypothesi, Gregory of Cyprus’ expression is a truth, and is therefore at least compatible with the truths expressed by the Symbol of 381. Interpolating such a phrase would “break the bond of love in the Church” only if, unrecognized by many as expressing a truth belonging to the deposit of faith, the phrase were nonetheless imposed on the Church as a whole, without general agreement, by formal addition to the Creed. But if the phrase were to be recognized by the Church as a whole as expressing a truth, what would be the problem?
Even if the Fathers of Constantinople I did intend to rule out any formal amplification of their Symbol via interpolation of some phrase expressing an agreed-upon truth of the Faith, I do not believe that such a stricture binds all subsequent ecumenical councils (and, as a Catholic, I would also deny that it binds the papacy). That is because the question whether a given creed should be formally amplified by a truth expressing an aspect of the Faith is not, itself, a doctrinal question but a disciplinary one. That a particular creed should or should not be formally amplified by a truth is not a matter belonging to the deposit of faith. It is a matter of deciding which is best for the unity of the Church in a particular set of historical circumstances. That is why, as I’ve said many times, my objection to the papacy’s 11th-century interpolation of the filioque into the Creed of 381 is pastoral rather than doctrinal. The interpolation did not introduce a falsehood; rather, it broke the bonds of love in the Church as a whole by professing, as the faith of the Church as a whole, a truth that was not carefully enough formulated to be recognized by the Church as a whole as a truth in the sense intended.
But it is also for pastoral reasons that I reject the proposal simply to delete the filioque from the Latin Church’s creed. Of course I would not object to such a deletion in principle as an ecumenical gesture meant to reverse the Roman error of 1014. If Rome made such a move tomorrow, my faith would not be affected in the slightest, nor would the Catholic faith be compromised in principle. The filioque has, after all, been formally defined as a dogma by more than one council whose decrees were ratified by Rome as binding on the Church as a whole; and in the course of discussing that dogma in several of my previous posts, I have taken it for granted as an article of the divine and Catholic Faith. Logically, none of that would be affected by the ecumenical gesture of deleting the mere phrase from the Creed. But I say that only as a theology geek. I am also a lifelong, practicing Catholic who came of age in the decade following Vatican II. I know the mentality of Catholics, especially American Catholics, very well. Given as much, I would strongly advise against Rome's making such a move as a pastoral matter. I do not suspect, I know, what would happen in the Catholic Church if such a move were made.
A millennium has now passed in which Catholics have simply taken the filioque for granted as a truth of the Faith. They recite it every Sunday at Mass without a second thought. But only forty-odd years have passed since the close of Vatican II. During that time, many Catholics of my and the previous generation got the idea that everything is basically up for grabs in the Church, that even dogma is a kind of extended policy paper which the next administration in Rome might rewrite if and when it suits them. That idea is false, and deeply so; but I'm afraid that the average Catholic, who is not a theology geek, can be forgiven for holding it. For in the 25 years or so after Vatican II, many priests and theologians held it too, and taught accordingly. Some still do hold it. Of course, after 26 years of John Paul II and three of Benedict XVI, they are now a defensive minority. But if the filioque were dropped from the Latin-Church creed, at least within my lifetime, those guys would be right back in business. They would succeed in reinforcing, in the mind of the average non-geek, the idea that everything is up for grabs. After all, the thought would go, if even the Creed can be changed by subtraction, then what couldn't be? The resulting confessional chaos would make the fights over birth control and women's ordination look like garden-party repartee.
Let me hasten to add that there would be nothing logical about such a development. I've already explained why, logically speaking, nothing substantive about the Catholic faith would thereby be changed. But right now I'm not talking about the logic a theologian can savor in his study; I'm talking about the mass psychology of average believers. I shudder even at the thought of confronting that if the filioque were dropped from the Creed.
What I'd like to see instead is unlikely to happen for a long time, but could happen in principle: an ecumenical council of East and West in which the Symbol of 381 is amplified in such a way as to exhibit the harmony between the filioque, properly understood, and the original Symbol. To get to that point, of course, much will have to change by way of ethos. Any such changes will be glacial. But when it comes to issues of dogma, glacial change is almost always preferable.