St John's 'Fount of Knowledge' is the first real 'summa' of Orthodox Christian theology. Born less than fifty years after the traditional date of the death of Muhammad, John was a powerful Confessor and Defender of Orthodoxy against the Heresy of Islam, which he also referred to as the "superstition of the Ishmaelites," calling Muhammad a "false prophet," and the Koran, made up of "ridiculous compositions."
All Christians — especially the Orthodox — should be familiar with the writings, life and witness of this great saint. See these posts to learn more...
St John Damascene's 'Critique of Islam'
Life of St John of Damascus
Troparion — Tone 8
Champion of Orthodoxy, teacher of purity and of true worship, / the enlightener of the universe and the adornment of hierarchs: / all-wise father John, your teachings have gleamed with light upon all things. / Intercede before Christ God to save our souls.
Kontakion — Tone 4
Let us sing praises to John, worthy of great honor, / the composer of hymns, the star and teacher of the Church, the defender of her doctrines: / through the might of the Lord’s Cross he overcame heretical error / and as a fervent intercessor before God / he entreats that forgiveness of sins may be granted to all.