Many people wonder if any Christian teaching seems more disconnected from "real life" than the doctrine of the Trinity. Yet the truth is that the Trinitarian creeds are the best "real life" map of God we have.
Jesus asked, "Who do you say I am?" and Peter got the answer right when he declared "You are the Christ, the Son of the Living God." However, Peter and the apostles, not being immortal, didn't stick around to address the million and one wrong answers which popped up during succeeding centuries--answers which, in one way or another, had at their heart the denial of Simon Peter's declaration. So the apostles appointed bishops whose job was to "guard the deposit of faith" entrusted to them (2 Timothy 1:14). The job of the bishops was to act as a sort of spiritual Food and Drug Administration, keeping the formula for the gospel medicine pure and helping unwary people tell the difference between the genuine and imitation brand gospels. Why? Because all the imitations looked real enough, but they put in a little too much of this or took out a little too much of that. And the result was poison.
Thus, when some said Jesus wasn't human and didn't die on the Cross, the Church said no to this since 1) the Apostles were quite clear about Jesus' humanity and 2) if Jesus didn't die then he didn't rise again and the whole gospel is utter nonsense. Others claimed Jesus was a good God and the Old Testament God was a bad God. The Church replied, "Then Who was Jesus calling 'Father' if not the Old Testament God of Israel?" Some taught Jesus was an angel. The Church pointed to the Book of Hebrews (which explicitly says he isn't) and deep-sixed that notion. And so the theorizing about Jesus went on for centuries with the Church continually acting as FDA in the face of well-meaning (and sometimes not so well-meaning) attempts to make the apostolic message "more palatable."
Then one day a guy named Arius came up with a theory about Jesus which sounded great. Jesus, said Arius, was God's first and greatest creation. He was a supernatural being (sort of like a godlet or an angel) made before anything else and, for our sakes, became human and died on the Cross. He is so far above us and so great that we could call him a "son of God", just as angels are called sons of God in the book of Job.
But, said Arius, that is not to say Jesus (or the Holy Spirit) is divine the same way God the Father is. Rather, he concluded, when John says the Son (also known as the Word or Logos) "was God" it means that he was strongly identified with God just as angels in the Old Testament are sometimes addressed as God by us humans. When it says the Word was with God in the beginning, he means he was with God when God began to create the rest of universe, not that he is eternal like God the Father is. In short, said Arius, the Father has always existed. The Son hasn't. And in plain Greek, that meant Jesus is not God.
In response, the Church observed, among other things, that Jesus claimed to forgive the sins of total strangers (which "God alone" can do [Mark 2:7]), was understood by the all the Churches from the get-go to be fully God and not some nuanced godlet (John 1:1), and loudly asserted he will judge the whole world at the end of time (rather like God) (Matthew 25). So he was either Not Nice, Not Playing With a Full Deck or... he was "the Christ, the Son of the Living God" who came (as he and his apostles said) to die for our sins and give us the eternal life of God.
Following this rather strong apostolic lead, the Church looked into its own past and noted that it had clearly proclaimed for centuries that God, the Big G himself and not some godlet had died on the cross for us at Calvary. It was okay, said the Church, to try to understand how Jesus could pray to God when he was God. It was okay to come up with explanations for Jesus' mysterious comment, "The Father is greater than I." But it was not kosher to do so by explaining away Jesus' deity. And it formulated this intellectually balanced view in the thing we call the Creed.
Yeah, but what about our hearts? Intellectual balance is all well and good, but how does it feed the hunger of the human soul? Isn't the essence of Christianity the simple truth that God is Love?
To this the Church replies, "What sort of relationship does the Father have with the Son? a love relationship! And the fusion of their love is so intense that out of it proceeds a Spirit of Love who is himself God: the Holy Spirit. This fusion of love between the three Persons of the Father, Son and Holy Spirit is what the One God is. Get rid of this love relationship (the truth of which the Creed was written to safeguard for us) and make God "simple" and you unfortunately get rid of the beautiful truth that God is love.
Which brings us back to the real world of looking for love in all the wrong places. For we do such things, as St. Augustine observed long ago, precisely because our hearts were made to be filled with God. It is by being swept up in the Dance of Love that is the Trinity, by being in Christ the Beloved, that we experience the Father saying to us, "My child, you are always with me and everything I have is yours." It is only in having God to offer back to God that we can fulfill all that a heart of love longs to offer and all that a God of love deserves. Yet if, as Arius said, Jesus and the Holy Spirit are just critters (even great critters) and not God, we're left empty-hearted and empty-handed because even they cannot give what they do not have: God's own eternal life. Yet this is exactly what Jesus promises. Therefore, if the gospel message is to be true to our hearts as well as our heads, it can declare Jesus Christ nothing less than "God from God, Light from light, true God from God, begotten, not made, one in being with the Father". And it can say nothing less of the Holy Spirit than that he is "the Lord, the Giver of Life" Who, with the Father and Son "is worshipped and glorified."
So in the end, the Church has done the only the thing She could, given the facts. In the language of theology, She proclaimed the strange dogma of a God who is Three Persons in One Divine Nature, neither confounding the Persons nor dividing the Substance. But those who speak the language of love know this is only another way of stating the simplest, most glorious truth in the universe: God is Love.