et al,
The Trinity is, more or less, Church Dogma.

It is a concept that that predates the Bible, at it includes the "Son;" presumably Jesus Christ.
I have had many people try to explain it to me. Essentially, it amounts to three distinct aspect forms or manifestations of one Supreme Being. This is, what apears to be, one of those mystical somethings taken on faith.
As a child, I was raised Catholic. I went to Catechism classes and was an Alter Boy. I guess the teaching was beyond me. Just as I recognized that the apparitions, immortals, and deities depicted on Chiller Theater were imaginary, so did I come to believe that the rituals, dogma, and devine teachings of The Church were mental gymnastics; invented by man, to foster and perpetuate a religious following and power.
There is a strong opposing view, based on faith and "selected" ancient writings that many believe were inspired by "God." This is, no less valid, than my evaluation. There are many that have the education and can speak for the Church, and have an absolute belief. I am often impressed and enjoy their lectures. But I am not persuaded. But I am just one man with an opinion.
Throughout history, there have been a huge number of those that were inspired by "God" for the benefit of man - in one way or another.
And there were many more. They all had followers. Some large groups and some very small. But the key to them all was their ability to attract people into the fold.
Organizationally, there are literally thousands of belief programs with many different agendas.
Of course my very favorite is The Illuminati. Periodically, someone tries to generate re-interest in The Illuminati; as Wes Penre does in his quest to enlighten people.
Most Respectfully,
R