(here’s an explanation that I gave on a different thread):
The fundamental difference between Islam and christianity is its disbelief in the Trinity. I would focus on this key difference when evangelizing muslims. It also doesn’t help that very few Catholics can actually explain the Trinity… Most muslims think our belief in the Trinity is absurd (ie. we believe in 3 gods, etc…) and if you can not only deconstruct this misconception but actually propose a very convincing and logical argument in favor of the trinity, I think you can definitely turn some heads.
Here’s how I would explain the trinity (drawing heavily from Michael Voris’ and Archbishop Fulton Sheen’s explanations):
First, we’re made in the image and likenss of God, so explaining the Trinity must start by illustrating that we, ourselves, are actually limited and imperfect versions of the Trinity.
For example, we each have a mind that contains both an intellect as well as thoughts. Our thoughts are created by our intellect and, together, they make up who we are. While our thoughts and our intellects are a part of one complete mind, they are separate entities in and of themselves… For example, if our thoughts and our intellects were the same thing then how could our intellect reflect on the thoughts that it creates? (ie. even an eyeball needs to have a mirror to see it’s own reflection) In short, there must exist some separation between our thoughts and our intellect even though the two are a part of one whole mind (you can see where I am going with this).
The next step is to show the fact-simile between our own minds and the Trinity.
There are two key differences that exist between our minds and the Holy Trinity. First, the Trinity is infinite, and secondly, the Trinity is perfect. Our thoughts have limits, both in terms of how much content they can contain as well as the “correctness” of the content contained within the thought. We are constantly learning new things as well as forgetting (or losing access to) past experiences. As a result, our intellect needs to reflect on thousands and thousands of different thoughts over the course of a day.
This is not the case with God. In fact, God’s intellect is so perfect that the thoughts that He creates are infinite both in their content and their correctness. In fact, these thoughts are so perfect that you would only actually ever need to have one of these thoughts (you would never need a second thought from such an Intellect, since that would presuppose that there was something lacking or something incorrect about the first Thought). This singular thought would contain every moral truth, every physical and mathematical truth, the parameters for the creation of the universe, our own DNA, everything that ever has been, is now, or ever will be would be contained in this thought. We christians call this infinitely truthful thought “the Son” and the intellect that created this thought, “the Father”.
This is what Jesus meant when He said that He was the word made flesh (He was this infinitely truthful thought, made into the form of a man). This is also what Jesus meant when He said that the Father does nothing except through the Son. If you want to raise your left arm, what does your intellect do? Well, it first creates a thought that contains the necessary instructions for movement. It’s the same for God, any action that the Father decides to bring about on the earth comes through the context of His infinitely truthful thought (that is, the Son).
So if God only ever had one thought, then what does He spend the rest of His time doing?
Well, put simply, the Father is in a constant state of reflection on this singular perfect thought (the Son). Think about how you feel after you solve a really difficult math problem… you see beauty in the truth that you have written down on paper. You would defend this truth if somebody tried to question it. In every sense of the word, you love this Truth (that is, you give yourself fully to it). In the case of God, however, the Truth that the Father has created in the form of the Son is so vastly superior to any truth that we could come up with that He Loves this thought infinitely more then we could possibly imagine. In short, the Father spends the rest of eternity infinitely loving the Son and pondering the beauty of the truth contained within the Son. This love that exists between the Father and the Son is what we call the Holy Spirit.
After explaining the Trinity, a good question for a muslim would be: If you deny that God exists as a Trinity, then exactly which part of the Trinity are you denying?
If you deny the Son, then you are saying that God exists purely as an intellect which is incapable of infinite and perfect thought. If God is incapable of perfect and infinite thought, then that means that God is capable of chaning His mind about things… this begs the question: if God can change His mind about things, then how do you know what God is thinking right now? How do you know that the Koran is correct at all, and that God hasn’t changed His mind completely since it was written??? Furthermore, in order for an intellect to change it’s mind about anything, it must either learn something new or forget something… how could God possibly do either of these things?
Similarly, if you deny the Father, then you are suggesting that God exists only as a thought. This is a problem, in and of itself, because if God is just a thought then something else must have created that thought (a thought is, by definition, created by an intellect).
I would try sharing these explanations and questions with your Muslim friends. At the very least, it might clear up any misconceptions they might have about Catholicism and get them seriously questioning some key issues between our two faiths.