Gennadius II was, together with his mentor, Mark of Ephesus, involved in the Council of Florence which aimed to end the schism between the Orthodox and Catholic churches.

[3] Following his tutelage under the famous John Chortasmenos ("didaskalos" of the Patriarchal School), Manuel-Mark might have then recommended him to study under his own previous master, Gemistos Plethon (d. 1452–1454), c.

In fact, in 1437 – in anticipation of the Council of Florence – the emperor formally studied Neilos Kabasilas's works along with Mark of Ephesus and Gennadius II.

From this, the Catholic Encyclopedia speculates that Gennadius was likely writing an academic exercise to inform his former master that Thomas Aquinas' opinions did not constitute a universally Latin approach to questions on the Trinity.

[citation needed] At the same council appeared the celebrated Platonist, Gemistos Plethon, the most powerful opponent of the then dominant Aristotelianism, and consequently an antagonist of Gennadius.

In church matters, as in philosophy, the two were opposed – Plethon advocated a partial return to Greek paganism in the form of a syncretic union between Christianity and Zoroastrianism;[citation needed] while Gennadius, more cautious, pressed the necessity for ecclesiastical union with Rome on doctrinal grounds, and was instrumental in drawing up a form which from its vagueness and ambiguity might be accepted by both parties.

It was at just about this time (1444) that he began to draw attention to the putative heterodoxy of Aquinas' "distinction of reason" between the attributes (viz., energies) and essence of God.

First, as contained in Martin Jugie's edition of his opera omnia, Gennadius interrupts chapters 94–96 of his discourse "On Being and Essence" of Thomas Aquinas and replaces the Thomistic explanation with that of Scotism in order to agree better with Gregory Palamas.

Marcus Plested observes that Gennadius' "love and esteem for Thomas was to continue undimmed throughout his career although he would often accentuate the note of caution in later works".

[11] He wrote many works to defend his new convictions, which differ so much from the earlier conciliatory ones that Leo Allatius thought there must be two people of the same name;[12] to whom Edward Gibbon: "Eusèbe Renaudot has restored the identity of his person, and the duplicity of his character".

In administering his new conquest, 21-year-old conquering Sultan Mehmed II wished to assure the loyalty of the Greek population and above all avoid them appealing to the Roman Catholics for liberation, potentially sparking a new round of Crusades.

Mehmed II conferred upon the new Patriarch the title of millet Başkanı' (Head of the Nation), thereby authorising him to oversee all matters concerning his co-religionists.

In a gesture reminiscent of the practices of Byzantine emperors, Mehmed II personally presented the Patriarch with the crosier and crown, symbols of his ecclesiastical authority.

This act of investiture, accompanied by Janissary guards, safely paraded the new Patriarch through the city, introducing him to the populace and marking a seamless continuation of the ceremonial traditions of the Byzantine Empire into the Ottoman era.

The city's famous patriarchal basilica, the Hagia Sophia, had already been converted into a mosque by the conquerors, so Gennadius II established his seat at the Church of the Holy Apostles.

Three years later the edifice, which was in a dilapidated state (in 1461 it was demolished by the Ottomans to make way for the Fatih Mosque),[17] was abandoned by the Patriarch, who moved again to the Church of the Pammakaristos.

[citation needed] In the spring of 1454 he was consecrated by the metropolitan of Heraclea Perinthus, but, since both the Church of Hagia Sophia and the palace of the patriarch were now in the hands of the Ottomans, he took up his residence successively in two monasteries of the city.

[citation needed] While holding the episcopal office Gennadius II drew up, apparently for the use of Mehmed, a confession or exposition of the Christian faith, which was translated into Turkish by Ahmed, Qadi of Berrhoea (and first printed in Greek and Latin by A. Brassicanus at Vienna in 1530).

The full reason for this step commonly attributed to his disappointment at the sultan's treatment of Christians, though Mehmed II seems to have kept the fairly tolerant conditions he had allowed to them; various writers hint at other motives.

Marcus Plested observes that Gennadius II's "love and esteem for Thomas was to continued undimmed throughout his career although he would often accentuate the note of caution in later works".

From this point of view, he stands at the head of a new period in the history of his Church; the principles that regulated the condition of Orthodox Christians in the Turkish Empire are the result of Mehmed II's arrangement with him.

The chief works of this time are the "speeches" made at the Council of Florence,[24] also a number of letters addressed to various friends, bishops, and statesmen, mostly unedited.

Marcus Plested observes that Gennadius II's "love and esteem for Thomas was to continue undimmed throughout his career although he would often accentuate the note of caution in later works".

[30] His most important work is easily his "Confession" (Ekthesis tes pisteos ton orthodoxon christianon, generally known as Homologia tou Gennadiou) addressed to Mehmed II.

Either for the same reason or to spare Muslim susceptibility he avoids the word Prosopa in explaining the Trinity, speaking of the three Persons as idiomata "which we call Hypostases".