Explanation: The concept of the "Trinity" - that the Father, Son, and Holy Spirit are one being, but also three separate beings - is a longstanding concept in Christian theology. As Christianity is intensely focused on orthodoxy - that one must believe the correct thing, not just act correctly - getting it 'right' has the importance other faiths might put on, say, oathbreaking, or murder. If you believe sufficiently incorrectly, you may have your soul damned to eternal torment, no matter what kind of life you led!

This concept of the Trinity is not always... intuitive to people who have not been raised in the tradition, and extremely small differences ("They are one now, but there was a time when the Father was lone") can be grounds for a quite literal inquisition burning you at the stake. Christians have spent about as much time as Christianity has existed killing one another over theological disputes of this sort. Heresy damns souls, after all!

... be careful how you word your religious metaphors...

"That's Modalism, Patrick!"

Obligatory link for this topic: https://youtu.be/KQLfgaUoQCw

It also helps that the term "Trinity" is nowhere in the Bible of course. (This coming from a Christian)