Though I firmly believe it, the doctrine of the Trinity has always confused me, and I suspect it has been a major source of misunderstanding between Christians and those of other religions.
First we Christians say that there is one God and only one God and there never has been more than one God and there never will be more than one God. Which all seems pretty clear to everybody. But then we turn around and start saying that the Father is God and the Son is God and the Holy Spirit is God.
At which our befuddled listener may ask...
Are Father, Son, and Holy Spirit just different names for the one and only God?
Uh, no, we say. They are all separate persons.
Do you mean Father, Son and Holy Spirit are three parts of the one God?
No. All three are fully God. You can't divide God into parts.
This, of course, is likely to cause a first class case of confusion, or worse, give the impression that somewhere along the line we've been lying.
I have heard people try to explain away this confusion, but I've never heard a good explanation until recently, when I came upon one by Anselm - long-time-ago archbishop of Canterbury - that I thought was quite good.
Interestingly, Anselm's argument for the Trinity relies upon the strongest affirmation that there is only one unchanging, eternal, indivisible, supreme God.
God, Anselm says, cannot be divided into parts. Why? Because if you divided God into equal parts, he wouldn't be supreme. Supreme means "above all else," so if God had an equal - or equals - he wouldn't be supreme. Only one can be supreme. But maybe you could divide God into unequal parts; say, 70 percent and 30 percent? Doesn't work. 70 percent of infinity is infinity. 30 percent of infinity is infinity. And infinity equals infinity, so you're back to having equal parts. If God is supreme and infinite, he simply cannot be divided.
Nor, he adds, can God be added to (Infinity plus a billion is what? Yup, still infinity.)
So, with that in mind, let's start with a question:
Can you imagine yourself?
Perhaps you saw yourself in the bathroom mirror this morning and can remember what you look like. You know how tall you are and how much you weigh and your beliefs and that your toenails need clipping. So if you close your eyes perhaps you can imagine yourself. Roughly.
Now consider God. Could God imagine himself? Of course! And not only could he imagine himself, he could imagine himself perfectly. But what would a perfect image of God be? A picture on the wall? A 3-D model? A spreadsheet of data about God? An angel? No. If God perfectly imagined Himself, the image would be... God. Anything less would an imperfect image.
Does this mean there are two Gods? Nope. There can't be. Infinity plus infinity is still infinity. Also, if God were to imagine himself as separate from himself, then his image of himself would be imperfect, because he is not separate from himself.
And so there you have the Father (the one imagining) and the Son (the one being imagined). Each of them is fully God, yet each is a different person.
Mind-twisting? Absolutely. But then so are a bunch of modern scientific notions, such as the curvature of space and the nature of light and the idea of electrons jumping from one orbit to another without passing through the intervening space and a bunch of other stuff. So get over it.
Between the Father and the Son there is also a relationship - a spirit, a Holy Spirit.
One might be tempted to say this relationship, or Holy Spirit, is a part of the Father and the Son, or a part of God, just as you might say that a relationship you have is a part of your life. But remember, God cannot be divided into parts.
So this relationship is not a part of God; it is God.
Thus we have the Father (who in our illustration imagines himself), we have the Son (the one who is imagined), and we have the Holy Spirit (the relationship between the Father and Son). One God - Father, Son and Holy Spirit.
In closing, I'd like to make two comments.
First, someone might say, "If the Father imagined the Son and the Holy Spirit resulted, doesn't that mean the Father came first?" No. God made time; he is not subject to it. "Before" and "after" and "first" and "second" and such terms have no meaning in regard to God. Also, Father, Son and Holy Spirit are all God, not parts of God. They all have the same infinite span of existance.
Second, maybe Anselm's explanation is in the neighborhood of the truth or maybe it isn't, or maybe I'm not explaining it well, but the critical thing isn't to know about God; the critical thing is to know God.