The Trinity—also called Triunity or three persons contained in one God—is who God is in his fullest united, one essence.
Let’s being with a popularizer of theology and philosophy.
C.. S. Lewis encourages us not to wish and beg for a shallow religion. In his book Mere Christianity, in the chapter “The Invasion” he discusses the Incarnation (God the Son becoming man), which can also apply to the Trinity.
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.
God is knowable as far as he has revealed himself through Jesus Christ, the Scriptures, the Holy Spirit who lives in us, and other avenues like the creation. However, no one has seen God in all of his full essence and being, except Jesus (John 1:18). We humans down here on earth are limited by our five senses and our finite minds, so how can we figure out and calculate the pure nature of God? “We can know God truly and sufficiently, but never comprehensively” (EDT, p. 343).
One reason that I believe in the Triunity is precisely because this doctrine is ultimately beyond my puny brain to figure out. I could never have invented it.
I simply have to accept Scripture as it is, even when I don’t understand all of it.
I trust God in the teachings I do understand (e.g. God loves me and saved me), and that leads me to trust him in the ones that I don’t understand completely (the Trinity).
It is best to go from known and simple things to the lesser known and difficult things. For example, it is best to study astrophysics and send out space ships over time before landing a human on the moon. Gradual. But unlike the moon landing, we will never understand the Triunity fully.
From simple and clear teachings to the deeper teachings is best the best route to travel.
To wrap this up, the deepest relationship between the three persons cannot be known by us in its fullest sense—it is not possible to contain the ocean in a thimble. But we can know him sufficient for our salvation and we can know him in truly—we know him in truth. But we cannot know him exhaustively—it would take eternity to accomplish it!
You already relate to the Father and the Son and the Holy Spirit. The articles on the Trinity at this website will help you put the doctrine together. Our God is Triunity—three persons in one essence, one God.
Get to know each person of the Trinity!
So how does knowing about the Trinity help me know God better?
There is an entire ten-point post that answers that question, here:
Written by James Malcolm
ARTICLES IN THE TRIUNITY SERIES
The Trinity: Why Would God Seem So Complicated?
Evangelical Dictionary of Theology, 3rd ed., ed. by Daniel J. Treier and Walter A. Elwell (Baker Academic, 2017).
C.. S. Lewis, Mere Christianity (Touchstone, 1943, 1996).