“To my knowledge, the Holy Trinity is a Catholic thing.”
No, actually the Trinity is the only way to go for people who believe in the bible and see Jesus as God incarnate.
The Trinity is supported by all main stream churches, Orthodox and all evangelicals and Pentecostals.
There are a few splinter groups who try to come up with an alternative (Church of God), but it doesn’t wash.
There are huge philosophical advantages to belief in the Trinity. It provides the basis for “unity in diversity” and it the basis of the three separate but equal branches of our government.
It is also the only basis for consciousness — which requires a three in oneness between the observer, the thing observed and the means of observing. Without a trinity of the mind thought would be impossible.
Here is an essay that I wrote on the Trinity. http://gillespiepoems.blogspot.com/2012/06/of-many-worlds-comments-on-nature-of.html
not all -- oneness pentecostals do not....