The most significant doctrinal difference between Islam and Christianity is the issue of trinity versus monotheism. The doctrine of trinity has a variety of interpretations, the most important of which are Arianism, Sabellianism, Orthodox, Deity, epiphany and the mysterious affair. These interpretations date back to the Old and New Testaments, or rather to religions preceding those scriptures. In all its interpretations, the trinity is strongly refuted by the Qur'an and Islam.On the contrary, Islamic monotheism maintains its identical sense however it may be looked into in the light of different accounts. In what follows, the author has taken a comparative look at the issue of trinity versus monotheism. From one side, he has criticized trinity, and he has clarified the deductive bases of monotheism in the light of theology, mysticism, and the Qur'an, from another side.