Christians believe in one God and only one God. But at the heart of Christianity is the belief that this one God is three Persons, the Father, and the Son, and the Holy Spirit. We count only one God. But when we count divine Persons, we count three.*
But notice that you can’t divide the one God into three more fundamental things, which make up the one God. God isn’t a compound of anything more fundamental. One is all there is, when it comes to God.
And yet you still have to count three. The three Persons of the Trinity are not really anything else. They aren’t roles of God, or modes of God. You can’t reduce the three Persons of the Trinity to some more fundamental something in order to get—at bottom—just one. In themselves, they are Persons, and there are three of them.
Some secularists suppose that everything whatsoever is reducible to elementary particles. For secularists, at the ultimate foundation of reality there is just the impersonal, the cold and uncaring bits of matter and energy that make up the material world.
What the doctrine of the Trinity tells us is just the opposite. At the ultimate foundation of reality, irreducible to anything else, there are the three Persons of the Trinity, who are one God.
There is a story told about an old lady who believed that the world is flat and rests on the back of a giant turtle. She was challenged with the following question, “Yes, but what does the turtle rest on?”, but the question didn’t trouble her. She just replied carelessly, “Oh, it’s turtles all the way down!”
She was wrong, of course, and the secularists are wrong, too. The whole cosmos rests on the loving and caring Persons of the Trinity. For Christians, it’s Persons all the way down.
This is a doctrine worth celebrating.