GP, the Orthodox Church doesn't look at the creed as an explanation and our opinion of the Trinity very much is a holy mystery. It's been revealed to us by the fathers and we hold to their explanations as divinely inspired. Only three fathers have the title of Theologian in the Orthodox Church - John (the Apostle), St Gregory Nazianzus, and St Symeon. Other fathers taught, and defended the faith against heresy (like st Athanasius, st basil, etc). We refuse to add, because it was given to us by people who were inspired by God to add explanation to defend the faith from heresy, and subsequently approved by councils (that is, the whole church).
Even the creed in general is thought of differently. It's not a list or statement of our belief, but a symbol of what we believe. It represents the faith, it describes it...its not exhaustive etc.
I had a little more time to sit down now, so I'll edit this to say:
1. The ecumenical councils were called to defend the faith. They do not exist to create conciliatory positions toward variance in opinion, theological or otherwise. They most certainly do not exist to act as ecclesiastical parliaments to decide "which pope is the real pope" which is what the Roman church was doing in the aftermath of the Council of Constance (at one point there were actually three concurrent popes...!).
2. Ultimately, though Basel-Ferrara-Florence wasn't a failed council because of who did or didn't vote, or who was or wasn't represented, or who did or didn't accept the vote, or even because the Orthodox present were under house arrest for fourteen years and signed onto "union" under what can only be called duress. It was a failed council because, quite simply, what it taught was not true. And how do we know? Because the church did not endorse it (see, alternately, the other councils considered Ecumenical).
3. And even if we say, for a moment, that Florence was a genuine and authentic union, if we assume that the Patriarchate of Constantinople works like Rome does (i.e., that if the chief bishop rules on something, it's binding on the whole church), then the union between Rome and the East was really only a union between Rome itself (the various parties in the divided west) Constantinople, and Russia (who was represented at the council by a Constantinople appointee). There was no union, then, with the Church of Jerusalem, Antioch, Alexandria, Serbia, Bulgaria, or Cyprus -- all of which were autocephalous churches in communion with each other (and Constantinople and Russia) at the time of the council. Again, this is hardly ecumenical.
4. And, again, even if we play along, and say the union was real, Russia had a very vocal and violent rejection of the union and in 1443 the three Patriarchs of Jerusalem, Alexandria, and Antioch met in Jerusalem and condemned the Council of Florence as "vile" and Patriarch Metrophanes of Constantinople as a heretic for accepting it.
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We say this in our churches on the Sunday of Orthodoxy (since 843) and this is the order of things.
"As the prophets have seen, as the apostles have taught, as the Church has received, as the teachers have set forth in dogmas, as the whole world has understood, as Grace has shone forth, as the truth was demonstrated, as falsehood was banished, as wisdom was emboldened, as Christ has awarded...This is the Faith of the apostles, this is the Faith of the fathers, this is the Faith of the Orthodox, this Faith has established the whole world."
The filioque is not part of the faith of the prophets, apostles, what the church received, what was dogmatized, etc.
The truth is, schism is as old as the church. It is a sad and unfortunate thing. But ultimately we don't hold to the church or humans or bishops for the truth, but Christ. This is the underlying mistake of the claims of the papacy.