The emergence of Islam in the Arabia in the first half of 7th century was certainly related to the Eastern Christianity there, and Muslims must have been familiar with the doctrines and rituals of their monotheist co-religionists, as the Qur’an often points out the problems of the Eastern Christian faith. After the establishment of the Islamic state, status of People of the Book (Christians and Jews especially), was thus turned into Dhimmis (protected people), who under the Muslim rule not only retained their religious traditions, but also had interactions with Muslims and served in the caliphate courts, so they played a significant role in the development of Islamic civilization and culture. As a result, dialogues between the Muslims and Christians in the course of Islamic history were continuously conducted no matter what conflicts or reconciliations took place. From the 7th to 15th centuries, the interaction between the two monotheist communities revealed the phenomenon of “one God in each’s worship” which reflects the common origins of Christianity and Islam. This article essays to draw a critical survey of the relations between Islam and Christianity in the classical and medieval Islam.