THE TRINITY IS TOO ABSTRACT TO ACCEPT; EVEN THE EARLY CHRISTIAN CHURCH BEFORE CONSTANTINE DID NOT BELIEVE IN IT.
It is wrong to assume that difficult beliefs must be unreasonable. Certainly the Trinity is a mystery, but why shouldn't it be? What's unreasonable about God, Who is mysterious to us, giving us a teaching about Himself which is difficult for our limited intellects to grasp fully? It is simply false to say the early Church did not believe in the Trinity. Many of the Church Fathers who lived well before Emperor Constantine and the Council of Nicaea of 325 AD (when the Trinity was precisely defined by the Church) explicitly use the word 'Trinity' to describe the Church's belief in a tri-person God. For example, Theophilus of Antioch wrote about the Trinity in 181 AD. Many early Christian doctrines were not precisely defined until they were attacked by heretics. The Didache (50-100 AD) tells Christians to baptize in the name of the Trinity just as Jesus commanded in Matthew 28: 19. The context of Matt 28: 19 points clearly to the 3 divine Persons of the Trinity; it forcefully indicates their unique oneness, their equality, and their distinct personhoods.
OK, SO EXPLAIN WHAT YOU KNOW OF THE TRINITY.
It can't be fully explained. But some light has been shone on this central Christian mystery, and what little we do know is more than enough for our tiny minds. Consider the following: The Trinity is the mystery of 3 Persons in one God. In the one divine nature there are 3 Persons: the Father, the Son and the Holy Spirit. No one of the Persons is either of the others, each is wholly Himself. The Father is God, the Son is God, the Holy Spirit is God. They are not 3 Gods but one God. Our one God (Who is infinite Beauty, Goodness and Truth) must be able to know Himself within Himself, and must be capable of a great love following His knowledge of the infinitely Beautiful, Good and True. But this knowledge and love within God must be identified with Him, yet in some way distinct from the source from which they proceed. The Father's knowledge must therefore eternally beget the Child of Infinite Intelligence Whom we call the Son, and it must give rise to a Spirit of Infinite Love that proceeds eternally from the love shared by the Father and the Son Whom we call the Holy Spirit. It has been revealed by Christ that in one God there is a Fatherhood, a Sonship and a Holy Spirit of Love; these 3 Personal Relationships within the one Divine Nature comprise the One Holy Trinity we are all called to adore.
* Please note that this text should be read in the context of the whole work and in recognition of the appropriate paragraphs of the Catechism of the Catholic Church highlighted in the index.