Since yesterday was Trinity Sunday, I thought it was a propos to talk about the Trinity for today's Catholic 101.
Every time a Catholic makes the Sign of the Cross, she's praying to the Holy Trinity. Every time a baby is baptized in a Christian church, it's done in the name of the Holy Trinity. The Trinity is one of the things that all Christians agree on--and if a denomination doesn't, it probably can't properly be called Christian. It's all over the creed and the bible. (This is one reason that some Christians say Mormons aren't Christian--they don't believe in the Trinity in the same way most Christians do.)
(Side note about the creed: The creed begins with "I believe"--so you really should believe everything you're saying, here. It's not something you should just mumble through, although I know I've been guilty of that. You really need to consent to believing the things you say you're believing!)
The Trinity is The Father, the Son, and the Holy Spirit--three Divine Persons, One God. Christians aren't polytheists. They are all one God, but three different Persons. St. Patrick famously used the image of a three-leaf clover to explain the Trinity to the Irish, and it's still a good example today, and one I've used in my classes.
Biblical evidence for the Trinity abounds, beginning in Genesis, when God says "Let us make man in our own image"--note the plural, there. Jesus tells his disciples to baptize in the name of the Father, the Son, and the Holy Spirit. We pray to the Trinity throughout Mass--every time we make the Sign of the Cross, for example.
The Trinity is easy to explain, but it can be hard to understand. A former professor of mine once said, "There are times when we must bend back the wings of our intellect, and bow before the mystery." The Trinity is a good place to do that. There's a story about St. Augustine, who once came upon a boy digging a hole on the beach.
"What are you doing?" he asked the child.
"I'm trying to bring the entire sea into this hole."
"That's impossible," St. Augustine said.
"It's no more impossible than you trying to fully understand the Trinity," the child said--and disappeared.
We can't understand how the Trinity "works", the same way we can understand how an equation in Algebra works, or a car engine, or a recipe. That doesn't mean we can't try to ponder it. Faith doesn't mean we just nod and smile and don't think about things. We're meant to have a faith that's muscular, and not just passive. But there are some things we will never understand fully--or at all.
However, if you'd like a diagram:
That's the other thing to remember--they're not the same. Each has a specific role--God the Father, the creator; God the Son, Jesus, the Redeemer, and God the Holy Spirit, the Sanctifier (one who makes things holy).
Trinitarian doctrine is incredibly rich, and I've just scratched the surface, here. But the main idea I impart to the kids I teach is who makes up the Trinity, and what those three persons do. And--we're not polytheists!