Muslim monotheism is vulnerable to many criticisms, particularly from a Christian perspective. Crucial is their rigid idea of absolute unity.

The Problem of Absolute Unity

Islamic monotheism is rigid and inflexible. Its view of God’s unity is so strong that it allows for no plurality at all in God. Hence, it sees nothing between monotheism and tritheism (three gods), and Christians are placed in the latter category. There are several reasons for this misunderstanding. For one thing there appears to be a misunderstanding of the biblical text related to God. Muslims also have a rather grossly anthropomorphic view of what it means for Christ to be a "Son" of God. This often seems to demand some kind of sexual generation, according to their thinking. But the terms "Father" and "Son" no more necessitate physical generation than the term alma mater implies that the school from which we were graduated was our physical womb. Paternity can be understood in more than a biological sense.

There is a deeper and more basic philosophical problem. In the final analysis God has no (knowable) essence or nature from which one can distinguish his three persons or centers of consciousness. This position is known as nominalism. God is absolute will, and absolute will must be absolutely one. A plurality of wills (persons) would make it impossible to have any absolute unity. And Muslims believe God is absolutely one (both from revelation and by reason). Reason informed Muhammad that unity is prior to plurality. As Plotinus put it several centuries earlier (205-70), all plurality is made up of unities. Thus unity is the most ultimate of all. Accepting this neoplatonic way of thinking leads logically to a denial of the possibility for any plurality of persons in God. Hence, by the very nature of his philosophical commitment to the kind of neo-Platonism prevalent in the Middle Ages, Islamic thought about God was solidified into an intractable singularity which allowed no form of trinitarianism.

This rigid monotheism is not entirely consistent with some of Islam’s own distinctions. Muslim scholars, consistent with certain teachings in the Qur’an, have made distinctions within God’s unity. For example, they believe the Qur’an is the eternal Word of God. Sura 85:21-22 declares, "Nay, this is a Glorious Qur’an, (Inscribed) in a Tablet Preserved! [in heaven]" And in sura 43:3-4, we read, "We have made it a Qur’an in Arabic, that ye may able to understand (and learn wisdom). And verily, it is in the Mother of the Book, in Our Presence, high (in dignity), full of wisdom" (cf. sura 13:39). This eternal original is the template of the earthly book we know as the Qur’an.

Muslims insist the true Qur’an in heaven is uncreated, and perfectly expresses the mind of God. Yet they acknowledge that the Qur’an is not identical to the essence of God. Some Muslim scholars even liken the Qur’an to the divine Logos view of Christ, held by orthodox Christians. As Professor Yusuf K. Ibish stated of the Qur’an, "It is not a book in the ordinary sense, nor is it comparable to the Bible, either the Old or New Testaments. It is an expression of Divine Will. If you want to compare it with anything in Christianity, you must compare it with Christ Himself." He adds, "Christ was the expression of the Divine among men, the revelation of the Divine Will. That is what the Qur’an is" (Waddy, 14).

Orthodox Islam describes the relation between God and the Qur’an by noting that speech is an eternal attribute of God, which as such is without beginning or intermission, exactly like His knowledge, His might, and other characteristics of His infinite being (see Golziher, 97). But if speech is an eternal attribute of God that is not identical to God but is somehow distinguishable from him, then does not this allow the very kind of plurality within unity which Christians claim for the Trinity? Thus, it would seem that the Islamic view of God’s absolute unity is, by their own distinction, not incompatible with Christian trinitarianism. The basic Muslim logic of either monotheism or polytheism is invalid. They themselves allow that something can be an eternal expression of God without being numerically identical to him. Thus, to use their own illustration, why can’t Christ be the eternal "expression of Divine Will" without being the same person as this Divine Will?

The Problem of Voluntarism

At the very basis of the Islamic view of God is a radical voluntarism and nominalism. For traditional Islam, properly speaking, God does not have an essence, at least not a knowable one. Rather he is Will. True enough, God is said to be just and loving, but he is not essentially just or loving. And he is merciful only because "He has imposed the law of mercy upon Himself" (sura 6:12). But since God is Absolute Will, had he chosen to be otherwise he would not be merciful. There is no nature or essence in God according to which he must act.

There are two basic problems with this radical nominalism: one metaphysical and one moral.

The metaphysical problem. The orthodox Islamic view of God claims, as we have seen, that God is an absolutely Necessary Being. He is self-existent, and he cannot not exist. But if God is by nature a necessary kind of being, then it is of his nature to exist. He must have a nature. Orthodox Islam believes that there are other essential attributes of God, such as, self-existence, uncreatedness, and eternality. But if these are all essential characteristics of God, then God must have an essence. Otherwise the attributes could not be essential. This is precisely how essence is defined, namely, as the essential attributes or characteristics of a being.

The moral problem. Islamic voluntarism poses a serious moral problem. If God is only will, without an essence, then he does not do things because they are right; rather they are right because he does them. God is arbitrary about what is right and wrong. He does not have to do good. He does not have to be loving to all; he could hate, if he chose to do so. Indeed, in sura 3:32 we read, "God will love you.... God is Oft-Forgiving, Most Merciful," but verse 33 says that "God loveth not those Who reject Faith." So love and mercy are not of the essence of God. God could choose not to be loving. This is why Muslim scholars have such difficulty with the question of God’s predestination.