Dear Rev. Know-it-all,
I have a non-Christian friend with whom I got into a big argument the other day. He is really offended by the idea of the Holy Trinity. He said that Jesus is not the Son of God, and belief in the Trinity is just too much to swallow. Jesus may be the Messiah, but He is just the Son of Mary and some other human being. He was only a messenger of God, if that. One should believe in God and His messengers, but there is no reason to believe that any of those prophets are God, only God is God. It is a contradiction of His transcendent majesty that He should have a Son. He went on to say that Christians have no right to call themselves monotheists (believers in one God) because they worship three Gods by claiming that God is a Trinity. I couldn’t answer him, so automatically I thought of you.
Yours,
Otto Mattock
Dear Otto,
Our difference with our fellow monotheists is not about the oneness of God, but about the nature of that oneness. The Trinity is a very reasonable idea if you believe what Jesus taught, that God is Love. Human beings can only long for perfect unity. God who is infinite can accomplish it. Who doesn’t want to be perfectly one with his or her spouse and their children? For us it is not possible, for God perfect diversity and perfect unity are possible if He is as, so many people say, absolutely sovereign. I remember meeting a holocaust survivor who was truly an amazing man. He was a good friend of a very dear nun who taught me Early Christian studies in grad school. She loved to bring her students to meet him and have him shake them up. We were having lunch when he looked at me and said, “You Christians! You say God has a Son. We Jews gave the world monotheism. This idea of God having a Son is step backwards to the religion of the Greeks and the Romans. God can’t have a Son!” I looked at him squarely and said who are you to say what God can and can’t do?” He was amazed. I was the first of Mother Mary Agnes’s students who had dared to challenge him. I hold to what I said.
Those who say that God is so absolutely sovereign that He can’t enter into real relationships effectually limit His sovereignty. If God wills to be relationship, then He can be. If God is love, true sacrificial self-giving love, then He reasonably has diversity within himself. If God is Love, then whom is He to love? If He IS love, but has only His creation as the object of His love, then He would be dependent on His creation for His very existence and would disappear along with the universe in a puff of logic! If you believe that God is love as Jesus of Nazareth taught, then God can be a Trinity, of Lover, Beloved and perfect Love itself, Father Son and Holy Ghost! The Trinity is a very reasonable idea, if (and only if) you believe what Jesus revealed, that God is love.
Belief in the Trinity also says a lot about humanity. Christians believe, as said by St. John Paul the Great, that God is the perfect family. Your family and mine attempt to be families, but God is family in its perfection. My destiny as a human being is to be adopted into that relationship which is God, the relationship that called all things into existence. The purpose for my existence and all existence is eternal and perfect love. It is the destiny, not the fate, of the universe and it is my destiny, should I choose to accept that destiny. 
The purpose of existence for the Christian is more than existence. The other monotheisms promise heaven, or at least the possibility of heaven. We Christians don’t just go to heaven. We go home to a Father who loves us and to a perfect family gathered from all time and space.  Even in this world, to believe in the Trinity means that we believe in the reasonableness of love. Spouses should be faithful, neighbors should be kind, parents should love their children and children should love their parents and the poor are our brothers. Life’s purpose and fulfillment is relationship, not just power and pleasure.
To believe that oneness of God excludes any real relationships is to isolate human beings and to demonize God. In his classic the Screwtape Letters, C. S. Lewis contrasts the absolute sovereignty of the Devil with the humility and self-giving of the Christian God. By “father below” he means the devil.
One must face the fact that all the talk about His love for men, and His service being perfect freedom, is not (as one would gladly believe) mere propaganda, but an appalling truth. He really does want to fill the universe with a lot of loathsome little replicas of Himself - creatures, whose life, on its miniature scale, will be qualitatively like His own, not because He has absorbed them but because their wills freely conform to His. We want cattle that can finally become food; He wants servants who can finally become sons. We want to suck in, He wants to give out. We are empty and would be filled; He is full and flows over. Our war aim is a world in which Our Father Below has drawn all other beings into himself: the Enemy wants a world full of beings united to Him but still distinct.
In other words, the devil insists that two things can be perfectly united only by one thing devouring the other. Two unique and separate things cannot be perfectly one and perfectly other. In our limited existence that may be true, but God who is absolutely and infinitely perfect can unite things which our limited power hold as completely separate, in other word for the devil, three cannot be one, unless one subjects the other two to its power. Unity must be a devourer “Your enemy the devil prowls around like a roaring lion looking for someone to devour.”  (1Peter 5:8) 
The Trinity is reasonable. To hold that the creator of the universe is anything less is actually quite unreasonable –and a bit disturbing.
Yours,
the Rev. Know-it-all