In the late 1300s through the mid 1400s, the Eastern Roman Empire (i.e. Byzantine Empire) was under constant threat from the Turks. Year after year, more and more territory was lost, church lands were confiscated by the infidels, hierarchy, priests and monastics were killed on mere suspicion of treachery against their new masters. The promises of the first Crusades were hollow resulting in the sack of Constantinople in 1204 by Western armies under Venetian leadership which set up a Latin Empire and a Latin Patriarchate. Three new Empires were carved out of that: the Nicaean, the Empire of Trebizond and the Despotate of Epirus. Even after the Latins were kicked out, the Nicaean Emperors were legally recognized as the Eastern Emperors but the territories controlled by Trebizond and Epirus were lost forever.
Ecclesiastically and culturally, the Empire was suffering. The Hesychast Controversy was eventually settled and St. Gregory Palamas was vindicated as was doctrine of the distinctions between energies and essence of God as well as the Hesychastic method of prayer. Still, with this triumph, there was cultural loss. Many Greeks, such as Bessarion (who later became a Catholic Cardinal) and Pletho (the foremost Platonist of his day) who were not in the Hesychast camp fled to Western Europe, often taking scholarly positions at Western Universities as well as all sorts of books and manuscripts from the ancient Greeks with them.
The Eastern Emperors, who were the only legitimate Roman Emperors, seeing the sunset of their kingdom did what they could to delay and even prevent the fall of their Empire. In the past, their skilled diplomatic maneuvers and arrangement of marriages had prevented invasions from the West and East. But time was no longer on the Emperors' side. In the past, aid was requested from the Western nations and also the Pope. But there was always a price tag attached: submission to the Pope and acceptance of various Latin doctrines.
Enter St. Mark of Ephesus. In one last ditch effort to gain military aid from the Western nations and have it sanctioned by the Pope, a council was convened at Florence in 1439. It was attended by John VIII Palaelogus, Emperor of New Rome, Pope Eugenius V, Patriarch Joseph of Constantinople, Metropoltian Mark of Ephesus, Bessarion of Nicaea, George Scholarius who would later become Patriarch of Constantinople (with the name Gennadius) amid many others. It was clear from the beginning that the Latins would have the upper hand. John VIII Palaelogus, so eager for military aid to defend his faltering kingdom, forbade the theologians in the Orthodox camp from arguing against the Latins for fear of offending them and, thus, not gaining the military hope he desperately needed. St. Mark would have nothing of it.
Despite being hamstrung, St. Mark consistently and forcibly defended the Holy Orthodox faith and decried all sorts of heresies propagated by the Roman Catholics such as the filioque, purgatory and the supremacy of the Pope. Though he was clearly getting the better of his Latin counterpart, John VIII Palaelogus silenced him and the Latin position was then, by default, proclaimed as the true expression of the faith. A decree of union was crafted. St. Mark, even in the presence of both allies and the schismatic Catholics, was the only one who refused to sign. He returned back to his See of Ephesus where he died in 1444 after a long bout with intestinal illness.
History was to vindicate St. Mark. Many of the Orthodox prelates who signed the decree of union were banished by their own flock because, in their eyes, these prelates had "sold out." For example, Isidore, Metropolitan of Kiev and all Russia, returned home to proclaim the union and commemorate the Pope during the Divine Liturgy. But the clergy and monastics, often who are the real defenders of Orthodoxy, rose up in rebellion against him and forced him into a monastery as a prisoner and had a new Metropolitan elected in his stead. The people even knew their faith was being sold out. The nature of the church being conciliar, the people are often the last defense against heresy and schism.
Thus, St. Mark's stand was the stand of True Orthodoxy. His stances were politically inconvenient and hostile to the Romans as they should have been. St. Mark should be our patron saint in our current world of ecumenism when the Orthodox are routinely called upon by other christian confessions to abandon the faith which Christ has left and has been preserved by us for the sake of unity. There can be no unity without unanimity of the faith. As St. Mark once said, "We seek and we pray for our return to that time when, being united, we spoke the same things and there was no schism between us." To be unified when so much still divides does no glory to Christ or His Church.
Holy Saint Mark, intercede before Christ's dread judgment seat that we may keep the faith as steadfastly as you!
By your profession of faith, O all-praised Mark--Troparion of St. Mark, Tone 4
The Church has found you to be a zealot for truth.
You fought for the teaching of the Fathers;
You cast down the darkness of boastful pride.
Intercede with Christ God to grant forgiveness to those who honor you!