A Brief Explanation of the Trinity - Still, though, would the true God exist in a complicated way like the Trinity?
We Christians believe that God is knowable as far as he has revealed himself through Jesus Christ, the Scriptures, and the Holy Spirit who lives in us, and other avenues like the creation. However, we also believe that God in his pure essence is unknowable. No one has seen God in all of his splendor and glory. We humans down here on earth are limited by our five senses and our finite minds, which process the data taken in by our five senses and somehow reformulate them into ideas. For centuries philosophers have been debating each other over how the mind works. So how can the human mind figure out and calculate the pure nature of God? Given God’s infinite greatness and ultimate unknowability, and given the finiteness of our five senses and the limits of our minds, it stands to reason, therefore, that some ideas about God are perplexing and unsolvable.
C. S. Lewis alludes to this in his book Mere Christianity, in the chapter "The Invasion" which discusses the Incarnation (God the Son becoming man). Here he does not accept the natural human wish that religion should be simple.
It is no good asking for a simple religion. After all, real things are not simple. They look simple, but they are not. The table I am sitting at looks simple: but ask a scientist to tell you what it is really made of—all about atoms and how the light waves rebound from them and hit my eye and what they do to the optic nerve and what it does to my brain—and, of course, you find that what we call "seeing a table" lands in mysteries and complications which you can hardly get to the end of.
Lewis uses the example of looking at a table, and if we let our vision go no further, then the table remains simple, but if we go down the path of knowledge any farther, then the table gets complicated really quickly. The same is true of our knowledge of God. If we keep our knowledge of him simple, then we are not bothered by the deeper doctrines. But once we probe more deeply, we discover that God is ultimately a mystery in his pure essence and glory.
Personally, one reason (among many) that I believe in the Trinity is precisely because this doctrine is ultimately beyond my puny brain to figure out. I could never have invented it. God in his pure essence cannot be known by me. He is beyond my five senses and my brain power, so it is logical that some ideas about him are difficult to understand.
For a great article on how the incomprehensibility of God (Deus absconditus) is a necessary criterion for establishing a true religion, go here. This article also discusses what the mystery of God means to us humans, with many biblical references. Finally, this article reinforces the idea that God is not simple.