It is not very difficult to find among the Orthodox on the internet the opinion that the witness of the saints is decidedly against ecumenism. Here is one example:
What the glorification among the Saints of our holy Father Justin emphasises is only something the Orthodox defenders of Tradition have known all along—we are supported by the witness of the Saints and Holy Fathers. Those holy men who have struggled in asceticism, the Elders, Confessors, and Martyrs of the Church, it is they that have denounced the Orthodox participation in the ecumenical movement. Its supporters, on the contrary, may have good people, ‘decent fellows’ among them, but they do not have Saints.
Another post, at On Behalf of All, gives quotes from saints who supposedly have fought against ecumenism: St. Photios the Great, St. Mark of Ephesus, St. Nikodemos the Hagiorite, St. Justin Popovich, and others.
This, however, seems to me to be a very distorted view of the tradition. Take St. Photios the Great, for instance. I think everyone knows that he was anti-Rome for its position on papal primacy and also considered the filioque heretical. But what is not so well known is his ecumenical relations with the Armenian “monophysites.” From Fr. Andrew Louth’s Greek East and Latin West: The Church, AD 681-1071, pp. 190-191:
The non-Chalcedonians [e.g., the Armenians] understood the Trisagion to be addressed to the Son and were fond of adding “who was crucified for us” to the exclamation “Holy God, Holy Strong, Holy Immortal,” while the Chalcedonians took the Trisagion to be addressed to the Trinity, and thus rejected the addition as heretical, implying that God suffered in his own being. The addition had been condemned at the Synod in Trullo. Photios’ long and careful letter confined itself to a detailed exposition of Christology, and made no mention of the matter of the Trisagion. All that was required of the Armenians was acceptance of Chalcedon. This letter was, however, part of a much more extensive exchange that took place in the 860s, culminating in a synod at Širakawan, which was followed up in the 880s, during Photios’ second patriarchate … What is striking about the exchanges is their irenic nature. Photios takes the line, as had John Damascene in the previous century, that “monophysitism” is not necessarily a heresy, but most often simply a confusion. The “monophysites” hold the same faith as the Orthodox, but reject the Definition of Chalcedon. Orthodox such as John Damascene, and later Photios, argued that “monophysites” rejected Chalcedon because they misunderstood it.” … There were further exchanges during the 880s, when Photios had resumed the patriarchate. These, too, were marked by a spirit of friendship, and a desire to set the question of dogmatic unity in the context of mutual love and respect. This in itself is worthy of record.
While the endeavor fell through, this still seems to show that the bold thesis that no saints have ever supported ecumenism is untenable. Let us consider another supposed anti-ecumenical saint, another Pillar of Orthodoxy even, St. Mark of Ephesus. St. Mark is well known to have called Rome heretical at the end of the Council of Florence and was one reason why the Council of Florence, which was set to reunify East and West, failed to be accepted in the East. Yet St. Mark began with great respect for the Latins at the council, calling them “most reverend” among other compliments. The council was apparently derailed by a nasty ex-Orthodox Dominican, Andrew Chrysoberges, who hurled abuses at the Greeks of the council, including St. Mark, as well as by other extremist Thomists who refused to compromise on any point with the Greeks (such as John Torquemada). In light of this, we see more clearly why St. Mark attacked the Latins as he did at the end of the council. Note, however, that it seems he only began referring to the West as heretics after he was hounded by their more ignorant and obstinate elements. Before this, he was quite happy with the ecumenical effort.
While I am not an expert on Orthodox history, I think it is fairly clear that the idea that Orthodox saints have always opposed ecumenism is far from the truth. On the contrary, many were very favorable of ecumenism, at least until non-Orthodox radicals put them off, which is only natural. I am very pro-Catholic, but it is because the Catholics I have known are very kind and fair to the East. But had I been alive in the early 1900s, when Catholics commonly claimed that St. Gregory Palamas was an incompetent, unwitting polytheist, I would have considered Rome heretical as well. Fortunately circumstances have changed, so recourse to the same old proof-texts for why ecumenism is bad will no longer work.