Mark of Ephesus (Greek: Μάρκος ὁ Ἐφέσιος, born Manuel Eugenikos) was a hesychast theologian of the late Palaiologan period of the Byzantine Empire who became famous for his rejection of the Council of Ferrara–Florence (1438–1439).

Mark was born Manuel in 1392 in Constantinople to George, sakellarios of Hagia Sophia, an Orthodox deacon, and Maria, the daughter of a devout doctor named Luke.

St Mark of Ephesus was one of the Eastern bishops who refused to sign the agreement with the Roman Church concerning the addition of the Filioque clause during the Council of Ferrara–Florence, however, was cordial in dialogues with the Catholics.

On his death bed, Mark implored Georgios Scholarios, his former pupil, who later became Patriarch Gennadius of Constantinople, to be careful of involvement with Western Christendom and to defend Orthodoxy.

In addition to canons and services to Jesus Christ, the Mother of God, and the angels, Mark honored his favorite Fathers of the Church: Gregory Palamas, John Damascene, Symeon Metaphrastes, along with a wealth of more ancient saints.

At the Council of Florence, the examination of this controversy had both text-critical and exegetical dimensions, as the participants debated the authenticity of sources, the precision of grammatical constructions, and the canon of authoritative patristic texts.

Echoing centuries of polemic, going back to Photius, the debates surrounding the Filioque admitted resonances of more recent discussions, such as those of John Bekkos and Gregory of Cyprus.

Mark's discourses at Florence are, in this regard, supplemented by writings which he produced in response to the Platonism of Gemisthus Plethon, who preached a radical identification of the human person with the soul, to the detriment of embodied life.

[4] The Palaiologan period in which Mark lived witnessed the translation of the works of Thomas Aquinas into Greek, an event whose repercussions have yet to be fully documented and expounded.

"[5] Also, eminent Eastern Orthodox scholars have already suggested that, during Mark's courteous and apologetic period of preparation for the Council of Florence, the Ephesine explicitly omitted condemnations of Aquinas by name, even if his two "Antirrhetics against Manuel Kalekas" contained condemnation of Thomistic Trinitarian theology, especially Thomas' implicit rejection of the essence–energies distinction and the Thomas' assertion that Aristotelean "habits" were very similar to the charisms or the Holy Spirit (e.g., Faith, Hope, & Charity).

[8] In effect, Mark of Ephesus' liberal use of Latin authorities paved the way for the fuller synthesis of his pupil Gennadius Scholarius after his master's death.

[9] Even if Mark was initially zealous for the "divine work of union" (τὸ θεῖον ἔργον τῆς εἰρήνης καὶ ἑνώσεως τῶν ἐκκλησιῶν)[10] in his opening speech to Pope Eugene IV, Mark was disgusted at the Latin efforts and plots to prevent him from reading the acts of the Ecumenical Councils aloud, wherein the canons prohibited additions to the Nicene Creed.

At the beginning of the council, in the same opening speech, Mark noted that two issues alone were necessary to overcome the historical divisions between the Latin and Greek Churches; namely, (1.)

In the end, Mark refused to sign the council documents following a debate with John of Montenero, who claimed that there was a difference in the dignity of persons in the Father, Son, and Holy Spirit.

Even if Mark dissented from Florence for traditional, theological motives he was quick to cite practical differences that emphasized the gulf between Latins and Greeks.