A major difference between Islam and Christianity, while both claim to be monotheistic, is the belief in the Trinity
According to Muslims:
Trinitarianism is not true monotheism. Christians believe that the Father is God, Jesus is God and the Holy Spirit is God. That makes 3 gods! Jesus is the Messiah but he cannot be God. Jesus is only a prophet, a human being, a messenger sent by God. The Bible says that Jesus prayed to God! If he was God then who was he praying to? He did not have the attributes of God such as omnipresence, omnipotence, timelessness or eternal being, omniscience, etc. Jesus never claimed to be God, not even according to the Bible!
The Short Response:
All the above arguments made against the Christian belief in a Trinitarian God are straw man arguments. Christianity also rejects the caricature of the Trinity that Muslims reject. It is likely that for many Muslims, their difficulty with accepting the Trinity roots from a lack of understanding of what the Christian doctrine of the Trinity is, and how it is textually substantiated from the Bible.
The Trinity Explained:
The word, "Trinity" does not occur anywhere in the Bible. It is a model which early Christian theologians thought up to be the culmination of 7 fundamental statements which are clearly taught in the Bible texts.
- There is ONE God.
- The Father is God.
- Jesus is God.
- The Holy Spirit is God.
- The Father is not Jesus or the Holy Spirit.
- Jesus is not the Father or the Holy Spirit.
- The Holy Spirit is not the Father or Jesus.
The Trinity is the concept of a SINGLE existential being, who exists in 3 distinct but perfectly united persons. Here, the word, "person" does NOT have anything to do with human characteristics but simply means that they are distinct from eachother.
For further reading about the doctrine of the Trinity, see:
http://www.thegospelcoalition.org/blogs/justintaylor/2013/04/24/what-do-we-mean-by-person-and-essence-in-the-doctrine-of-the-trinity/
At this point, it is normal for Unitarian Monotheists (such as Muslims) to respond that the entire concept of the Trinity cannot possibly make any logical sense. In other words, such a relationship where the persons of the Trinity are distinct from one another but are a single perfectly united existential being is a logical impossibility. The problem is that these same people who pose this objection accept a trinitarian view of humans without even realising it!
Imagine I was involved in a car accident and suffered very serious injury but was still conscious.
You could say "Diva has suffered serious injuries". In this case, "Diva" refers exclusively to my body. However we understand that my body IS
me. It isn't just an incomplete part of me. It is entirely me.
Now let us say that in my final waking moments, I say that I'm afraid and think I'm going to die. You could say "Diva is afraid and thinks he is going to die". Here, "Diva" refers exclusively to my mind. My mind is neither my body nor my soul but it IS entirely
me.
Eventually, let us imagine that I succumb to my wounds and die. At my funeral one may say, "Diva is no longer with us", in reference to my soul (or spirit) being entirely me. At that moment, it would be considered absolutely ridiculous for someone to object saying, "what are you talking about? He's right there in the coffin!". Yes, my body is there in the coffin and it IS
me but my soul which is ALSO
me is not there.
So, what we can observe is that we humans can be conceptualised as triune beings. It's nonsense to jump from this to accusing someone of believing that each individual human is actually 3 existential beings! No, not at all! A human can be said to be a single existential being who exists as 3 distinct but perfectly united members of a trinity - mind, body and soul (or spirit).
Dr. Zakir Naik's Challenge:
https://www.youtube.com/watch?v=u3Z-XFq8F_w
The man on stage is a prominent, well-known Muslim theologian and apologist. He travels all over the world to teach and give talks on Islam, always drawing huge crowds consisting of both Muslims and Non-Muslims.
The above video is one of several instances where Dr. Zakir Naik, in a conference talk, issues a challenge to a Christian audience member. To paraphrase, the challenge is to to show him where in the Bible, Jesus is recorded to claim that he is God. If he sees such a verse or passage in the Bible, Dr. Zakir Naik says he will become a Christian.
Well, Dr. Zakir Naik, challenge accepted. Here are two accounts from the Bible where Jesus claims to be God.
John 10: 30-33
30 I and the Father are one.”
31 Again his Jewish opponents picked up stones to stone him, 32 but Jesus said to them, “I have shown you many good works from the Father. For which of these do you stone me?”
33 “We are not stoning you for any good work,” they replied, “but for blasphemy, because you, a mere man, claim to be God.”
John 14: 8-11
8 Philip said, “Lord, show us the Father and that will be enough for us.”
9 Jesus answered: “Don’t you know me, Philip, even after I have been among you such a long time? Anyone who has seen me has seen the Father. How can you say, ‘Show us the Father’? 10 Don’t you believe that I am in the Father, and that the Father is in me? The words I say to you I do not speak on my own authority. Rather, it is the Father, living in me, who is doing his work. 11 Believe me when I say that I am in the Father and the Father is in me; or at least believe on the evidence of the works themselves.
In the first passage (John 10), even the Jewish listeners who understood the context of Jesus' words fully understood that this is what he meant and were ready to stone him. To interpret his sayings otherwise would be to claim to understand his Aramaic words better than those who were actually there and heard what he said in their own native language.
As for the second (John 14), it is difficult to interpret "anyone who has seen me has seen the Father" as anything other than a claim to divinity.
So, as long as Dr. Zakir Naik's challenge was sincere and not rhetorical, this should be sufficient to convince him.