I think the idea that God is simple (not made up of parts) is held in common by Islam, Judaism and Christianity.
Both you and Deb objected to the use of “irrational” so clearly that isn’t the right word. But it seems to me that you are definitely saying there is something different between saying that God the Good is beyond comprehension/mysterious, or God the One is beyond comprehension/mysterious, and saying that God the Trinity is beyond comprehension/mysterious. In the first two cases the language is meaningful, is coherent in itself and in relationship to each other, but we know because we are finite, that we cannot contain God. God is fundamentally beyond Being and Knowing. With the Trinity on the other hand, the language itself, from the very first, does not cohere. 3=1 (or something…I gave the 4 propositions in the previous post, for instance, which are just flat out contradictory). That seems to be a different kind of mystery than what is normally being invoked.
Or to put it slightly differently, there is a different kind of act of faith going on when one says God is One than when one says God is Trinity. You seem to be saying that the latter does not have a relation to us as thinking beings, where the first does. That is what it seems to me you are saying. Maybe?
Since God is infinite and we are finite with finite minds we can never fully comprehend or understand God. That does not mean we can have no understanding, but not complete understanding. We can know something about God.
We can start with the statement, God exists. Muslims and Christians agree that God exists. God is existance. He reveals Himself as existance to the Jews. “I am that I am.”
Man using his intellect alone can figure out that God exists. The Greeks did this. Beyond that if we are to know anything more about God it is through revelation. He reveals Himself to us and things about Himself. These things we call the infiinte perfections of God. God is love. God is life. God is truth. All of the perfections of God are found in each of them.
We know something about love. Love can not exist by itself or alone. There is a lover and the object of love, the beloved. So we have the lover, the beloved and the love, a trinity, so to speak. Love gives love to love. Love gives itself. If love does not give itself it is not love. The same is true of life. God is the giver of life. He gives life to Himself eternally.
In Christian theology we say God is three persons with one nature. The three persons are identical in nature, or one. They share the same nature. In order to understand this you must first make a distinction between person and nature. There is a difference between who you are and what you are. You are a human person, because you have human nature. You breathe, eat, walk, talk, see, hear, laugh, cry, pray, love, etc. You can do these things in your nature. A bird has bird nature and can fly. You can not fly.
I can do in general the same things you do, because I also have human nature. We share human nature, but we are not the same person.
My nature makes me what I am. That is not who I am. There is a difference between what I am and who I am.
God is infinite being. Divine nature is infinite. My nature is finite, or limited. You and I are not the same. We look different and are different in many ways. We have different levels of intelligence, height, faces, abilities, interests, etc. If we were two persons and were infinite, we would be identical. If one of us had something the other lacked the one lacking would not be infinite. But if we both were infinite we would be the same, identical in nature.
As love can not exist by itself, without an object of love, so knowledge can not exist alone. There is the knower and the known. God knows Himself. Another way of sayng this is God has an idea of Himself. He sees Himself. You could even say that He has an image of Himself. Since God is infinite this image or knowledge of Himself must also be infinite, lacking nothing that is God, or the image or idea that is known would not be complete knowledge.
This is very difficult to comprehend, and even more difficult to express verbally.
I am also known by God. God knows me. In fact my identity, who I am comes from God who made me. I am a created being, a creature, known by the being who made me. The most fundamental essence of who I am comes from God, who made me, knows me and loves me. I am the object of divine love, and so are you.
But God is uncreated Being. He is eternal. He knows and loves Himself in three infinite divine persons. Another way of saying that is God is three whos and one what. Infinity pours Infinity into Infinity. God is not stagnant, bottled up within Himself. He reveals Himself to us in Three Persons. This happened first at Jesus’s baptism by John in the Jordan.
But you ask how the doctrine helps us. The answer has to do with what God is doing with us, why He made us, what we are becoming.
God took on our nature. He could be with us, teach us, talk with us and tell us about Himself. We are made for Him. He shares our nature. He will raise us to share His. So Saint Peter writes that we will participate in divine nature.
You are a Muslim and these things are not part of your faith. As with all revelation of things we can not figure out on our own, we will either accept them in faith and have knowledge of them or reject them. In this case is it a matter of knowing why we exist.
It spoke about multiple universes and other things that I did not understand. Simply because my level of comprehension is not great enough to fully understand the book does not mean that quantum physics is irrational.