As I was watching various videos by Muslim friends today and reading their posts on Facebook, something ironic dawned on me: when they say Jesus is not God because God cannot be a Trinity, it is because they are making God in man's image. Allow me to explain.
These Muslim friends will often question the viability of the Trinity, saying "what sense does it make that God can be three and one at the same time?" When they make this statement, it is sometimes because they have not understood the sense in which God is three and the sense in which He is one.
Surely they cannot be questioning whether something can be 3 in one sense and 1 in another? That is obvious even to a child. For example, a family can be three and one at the same time; one family can be made up of three people, and three people can make up one family. To be clear, I am not using this as an analogy to the Trinity. I'm simply demonstrating that even Muslims can understand and will agree that something can be 3 in one sense and 1 in another.
Now when it comes to God, the Trinitarian belief is that God is 3 in one sense and 1 in another; specifically, He is 3 in person yet 1 in being. The difference between a person and a being might have to be explained to a Muslim who says a Trinity is not viable; perhaps he will then desist from saying the Trinity is not viable. My experience, though, is that he won't.
Why, you ask? I'll tell you why. Because they usually conclude that a being can only have one person. The idea that the nature of God's being might be more complex than that of man's, that God in His infiniteness can certainly contain more than one person in His being, simply is unacceptable to the average Muslim. The reason why? Because the Muslim has made God in man's image. He sees that man, and all of creation, is one in being and (at most) one in person. Since that is the nature of creation, that must be the nature of God.
Some might say "No, Muslims only deny the Trinity because the Qur'an says it's false!" If that simple fact were the case, then Muslims would disagree with Christians' view of God; however, they would not argue that the Trinity is absurd. As it is, they quite regularly argue the latter, and can only do so based on a presupposition: that he can only be one in being and one in person; that God's nature is that of man's.
In reality, as worshipers of an infinite God, we ought to consider God's nature a matter of revelation: if we are to know what His nature is like, He must tell us. We cannot presuppose the details about Him with any degree of certainty. Is He a Trinity? Is He a Unity? Is He a Hundredity? We cannot know unless He reveals it to us.
The next time one of your Muslim friends argues against the deity of Jesus based on the inviability of the Trinity, make it clear which one of you is creating God in man's image.