The Roman Catholic Church forced the Trinitarian belief on everyone under their power under the penalty of death for hundreds of years. This is why so many people have come to blindly believe in it. It has literally been forced and brainwashed into Christianity and now it has become a sacred cow.

I believe that the Catholic Church is directly responsible for the mass belief in the Trinity and every "denomination" following the trinitarian doctrine is essential tied to the Catholic Church, whether they think so or not.

That's my thoughts on it anyway.