One of the most common objections Christians encounter when engaging Muslims is the issue of the Trinity. Yet it is probably the one issue Muslims could not care less about, and they only use the objection to derail the average Christian into silence. But it is one of the easier objections to answer, and at least in concept get a Muslim agree to the possibility of a trinity. How?
Let's review the common Muslim response to the Trinity. Muslims believe the Trinity is a contradiction by suggesting something cannot exist as a single entity yet have multiple component pieces of that single entity. They will argue that 1+1+1=3 and suggest by this formula that Christians worship three Gods, not one.
First, I find it quite fascinating that Muslims will admit that God cannot be known. According to Islamic theology, God is so transcendent and so far removed from us that there is no way we can ever possibly hope to know God. But in the very next breath, the same Muslim will state with absolute certainty he KNOWS God is not one in three! If God cannot be known, then how can the Muslim KNOW this about God?
Second, I like to get Muslims to agree, at least in principle, that the concept the Trinity is intelligible and not a contradiction. I will ask a Muslim if they are comprised of only the chemical elements that make up their physical body. Every Muslim will agree that, as humans, we are more than just our physical bodies. When our flesh dies, our spirit or soul lives in eternity. The Qur'an says explicitly in Sura 3:169, "Think not of those who are killed in the Way of Allah as dead. Nay, they are alive, with their Lord, and they have provision." The Bible says in 2 Corinthians 5:8 that to be absent from the body is to be present with the Lord.
When our physical bodies die our spiritual nature continues. But since we have two natures as humans, does that mean we are two separate entities? If I have a body and spirit, does that make me two persons? Using the Muslim argument againt the trinity, it does, yet no Muslim will agree that he is two persons because he is composed of two natures: physical and spiritual. If we, as mere mortal human beings, can have two natures in one being, why is it so difficult to conceive an infinite God who has three natures, yet is one being?
Third, only the triune Godhead of the Bible can help us understand and make sense of our ability as humans to love one another. Recall that the Islamic concept of God is that he is absolutely and singularly one. Before he created mankind, he alone existed in absolute singularity. Now, think about the concept of love: Does it not require at least two entities to be intelligible? For love to exist, there must be both a lover and one on whom love is bestowed. The concept of love in absolute isolation from another is unintelligible. Yet Muslims will claim that love is one of Allah's attributes, but where did that love come from? In eternity past when Allah was all alone, whom did he love? The god of Islam cannot possess the attribute of love because he had nobody to love prior to our creation. So when Allah created man, how did he give man the ability to love if he himself did not possess such ability? How can god give us something he himself does not have?
However, the Trinitarian God of Christianity makes the concept of love perfectly intelligible. We humans love because God has always been loving. We have the ability to love because our God has always had the ability to love also. God gave us something He himself had: love. God has always existed in a loving relationship within the trinity: God the Father in a loving relationship with the Son, and the Son with the Spirit, and the Spirith with the Father. Our God is a loving God, has always been a loving God, and always will be a loving God. The god of Islam cannot make the same claim.
Remember, most Muslims have no genuine interest in understanding the trinity or even acknowledging the possibility of the trinity. When they raise the objection, answer it quickly, show him or her you have a response, nip their objection in the bud, and then move on to a more serious issue: our eternal salvation. Don't get side tracked by an innocuous objection and let the Muslim stop the conversation there. Keep moving on to serious matters.