The Doctrine of the Trinity (John 1:1)
The early Church councils (Nicaea 325 AD, Constantinople 381 AD, and Chalcedon 451 AD) can be reduced to this: “There is one God eternally existent in three Persons: Father, Son and Holy Spirit. The second Person, God the Son, has two Natures: Human and Divine”. As for the Trinity we can say, without equivocation: "Relational attributes in a non-contingent being presuppose that relationship is necessary in that being".
Every non-Trinitarian I have shared this with, once they grasp the concept begins to freak out because it underscores an inherent conflict that exists in all non-Trinitarian Monotheism. C. S. Lewis's friend, Charles Williams put it like this about the Muslims, "They denied love to God except by means of his creation". And Lewis wrote of the Christian understanding of God that, "In God all the concrete reciprocities of love exist from time in memoriam." Now for the questions: 1. If you ask a non-Trinitarian Monotheist if he or she believes God is a contingent being or a non-contingent being what would he or she say? That is, does God's existence depend on anything else or is he self-existent and necessary? If they understand the question my experience is that 100% of the time they will say, "Non-contingent." 2. If you then ask, "Do you believe God is a God of love?" They will answer "Yes." Since love is a relational attribute the question then arises: 3. Who is the object of God's love? If they answer we are; or his creation is. Then the theology of that person is highly suspect. On the one hand, if God needs His creation to fulfill His nature to love then God is a contingent being in need of something outside Himself. If this were the case he is not a non-contingent being but a contingent one. He needs his creation in order to express his love. On the other hand, if God is non-contingent and yet possesses relational attributes we must then assume that God is a being in whom a relationship exists as a necessary one. All non-Trinitarian Monotheists are reduced to nonsense at this point. And this is why I say: "Relational attributes in a non-contingent being presuppose that relationship is necessary in that being".
Any one may legitimately claim that there are mysteries in matters of faith. It stands to reason that the existence of an infinite God would be beyond the grasp of any kind of full comprehension by his creatures. We call these things mysteries of faith and expect that they are so.
Nevertheless, it is unwise to play the mystery card too quickly for all of us today comprehend some things that were beyond our grasp ten years ago. To claim a thing as a mystery too quickly may be an excuse for laziness of thought and investigation. Furthermore, it may cloak nonsense developing in one’s faith. There may be matters of faith that are supra-rational and beyond the grasp of reason but there should be nothing that is irrational about the faith. Contradictions lead to faith that is both nonsensical and preposterous. Christians have injunctions against such thinking. They are told to test the spirits to see if they are from God. They are told to beware of those who say, “Thus saith the Lord when the Lord hath not said.” Christians should be neither lazy not irrational about matters of faith.