[3][4] Philotheus I is commemorated on 11 October,[5][6][7] and is regarded as a "Protector of Orthodoxy", alongside Saints Photius I of Constantinople, Mark of Ephesus, and Gregory Palamas.

At Mount Athos, he lived his monastic life first at Vatopedi monastery, where he formed a relationship with Savvas the New of Kalymnos the Fool-For-Christ (d. 1350), for whom he became a biographer.

[8] He was a supporter of Saint Gregory Palamas and became a follower and advocate of the form of contemplative prayer called Hesychasm, and the Orthodox theology of uncreated Grace.

[3] As a writer of note, Philotheus wrote works on the theology of the Uncreated Energies of God and refuted the scholastic philosophy that was then current in the Western church.

[8] His most famous work, written in 1339,[6] was the Hagiorite Tome, the manifesto of the Athonite monks on how the saints partake of the Divine and uncreated Light that the Apostles beheld at the Transfiguration of Jesus.

[8][note 3] In August 1353, Philotheus I, renowned for his learning and his Orthodoxy, was appointed Patriarch of Constantinople by John VI Kantakouzenos.

In October 1369 John V, having travelled through Naples to Rome, formally converted to Catholicism in St. Peter's Basilica and recognised the Pope as supreme head of the Church.

[3] To this end, in 1367 he was in favour of holding an ecumenical union-council to resolve the differences with the Western Church,[1][6] however the discussions came to nothing as the idea was rejected by Pope Urban VI in 1369.

After some hesitation, Saint Sergius complied with this request, and the Trinity monastery, by adopting the Studite Constitution, became the model for all other late medieval Russian Koinonia.

[18] In 1375 Patriarch Philotheus I consecrated Cyprian as "Metropolitan of Kiev, Lithuania, and Russia" in the lifetime of Alexius, the lawful incumbent of two of these three sees.

[8] Robert F. Taft affirms that the liturgical codification of the Eucharistic service of the Great Church reached its full form in the diataxis of Philotheus I of Constantinople.