This article deals with the conflict between philosophy and religion, which has appeared in the course of the propogation of the ancient Greek philosophy in the Islamic world during the medieval period. It is said that Greek sciences started to be translated into Arabic from the latter half of the eighth century. The translation movement reached its climax during the reign of al-Ma'mūn, the caliph of the Abbasids in the 9th century, who has established the famous academic institution called 'House of Wisdom' to support the study of Greek sciences as a part of national policy. As a result, almost all the works of major philosophers of ancient Greece such as Plato and Aristotle have been translated into Arabic. It is also important to note that the works of Neoplatonic philosophers influenced very much on the rise of the medieval Islamic philosophy, which was represented by al-Fārābi and Ibn Sīnā. The Muslim Neoplatonists such as al-Fārābi and Ibn Sīnā have contributed a lot to the development of the Islamic philosophy with their new concepts about God, the origin of the universe, the creation of the world, and so on. But conservative theologians showed hostile attitudes to the Neoplatonists because their philosophical views were quite different from the traditional religious doctrines. The Muslim Neoplatonists tried to explain the process of generation of the world based on their unique emanation theory, which could not be compatible with the traditional theory of 'Creation out of Nothing(creatio ex nihilo)'. In this context, the 11th century Muslim theologian Al-Ghazāli criticized severly the Muslim Neoplatonists in his famous book Tahāfut al-Falāsifa(the Incoherence of the philosophers). This article is to reveal what was the underlying cause of al-Ghazali's attack on the philosophers and compare the views of the Neoplatonists and the theologians focusing on the topics such as the creation of the world, the concept of God, and the eternity of the matter.