Mark of Ephesus on the Latin Church, for those who say it's all politics or that the filioque is an excuse rather than a reason (both of which are Grade F baloney):
"The Latins are not only schismatics but heretics… we did not separate from them for any other reason other than the fact that they are heretics. This is precisely why we must not unite with them unless they dismiss the addition from the Creed
filioque and confess the Creed as we do.”
“These people admit with the Latins that the Holy Spirit proceeds and derives His existence from the Son. Yet, with us, they say the Spirit proceeds from the Father. The Latins imagine that this addition to the Creed is lawful and just, but we will not so much as pronounce it. They state that unleavened bread is the body of Christ, but we dare not communicate it. Is this not sufficient to exhibit that they came to the Latin council not to investigate the truth, which they once possessed and then betrayed, but simply to earn some gold and attain a false union? Behold, they read two Creeds as they did before. They perform two different liturgies – one on leavened and the other on unleavened bread. They perform two baptisms – one by triple immersion and the other by aspersion; one with Holy Chrism and the other without it. All our Orthodox customs are different from those of the Latins, including our fasts, Church rites, icons, and many other things. What sort of union is this then, when it has no external sign? How could they come together, each retaining his own?”
“‘But if,’ they say, ‘we had devised some middle ground between the dogmas (of the Papists and the Orthodox), then thanks to this we would have united with them and accomplished our business superbly, without at all having been forced to say anything except what corresponds to custom and has been handed down (by the Fathers).’ This is precisely the means by which many, from of old, have been deceived and persuaded to follow those who have led them off the steep precipice of impiety; believing that there is some middle ground between the two teachings that can reconcile obvious contradictions, they have been exposed to peril.”
(More here)
I find it very significant that the lone holdout from the Council of Florence would write such things. It seems that those who see nothing of substance in the Great Schism have probably not studied both sides' reasons for it with equal vigor. I know that the Latins also produced their polemics (such as Aquinas'
Contra Errores Graecorum, which I always thought was an odd title since it seems like it is mostly meaning to say that the Greeks really agree with the Latins, but just don't realize it, rather than delving too much into supposed Greek/EO "errors"), so a reading of those will help anyone who is seriously interested in this topic to understand both sides a bit better. When it comes to Latin claims against the Greeks, I find many of the specific charges to be rather spurious.