The great Church Fathers St. John of Damascus (8th c.) and St. Gregory Palamas (14th c.) clearly point out the inherent fallacies and inconsistencies of Islamic theology by articulating the dogma of the Holy Trinity as found in the scriptures, and comparing it to Quranic passages. St John Damascene is quite bold in his refutation:
“…We say to them: ‘As long as you say that Christ is the Word of God and Spirit [“The Messiah, Jesus son of Mary, was Allah’s word which he conveyed unto Mary, and a spirit from him” (Sura 4:171)], why do you accuse us of being Hetaeriasts [“Associators” – i.e., those who associate another with God]? For the word, and the spirit, is inseparable from that in which it naturally has existence. Therefore, if the Word of God is in God, then it is obvious that He is God.
“‘If, however, He is outside of God, then, according to you, God is without word and without spirit. Consequently, by avoiding the introduction of an associate with God you have mutilated Him. It would be far better for you to say that He has an associate than to mutilate Him, as if you were dealing with a stone or a piece of wood or some other inanimate object. Thus, you speak untruly when you call us Hetaeriasts; we retort by calling you ‘Mutilators of God.’ (St John of Damascus, Critique of Islam, emphasis added.)