Well, I'm not Christian so I'm not attached to the doctrine, but although it is complex and hard to understand, I don't get why it is so controversial. People who say it doesn't make sense- well, why would God make complete sense to us? He's God. Of course He is complex.
I think saying it makes no sense -- well the trinity
doesn't make sense to many. Me it makes my head spin. I've tried to get my head around that teaching in various ways. There have been a few comments on this forum during the months I've been here that have said things to make the trinity
almost graspable. So I appreciate those things. But I'm a long way from being able to embrace or defend the Trinity doctrine on my own.
But as far as why it's controversial and why people feel the need to complain that it doesn't make sense, the reason is as follows:
The reason comes from both how dogmatic the teaching of the Trinity is, and also history.
The teaching was controversial in early church history when some people said "trinity" and others said "nuh-uh" or "Hunh?" and anybody who didn't agree "trinity" was deemed a heretic and things got nasty.
Today, in most churches the classical dogma of the trinity is still adhered to. Most, possibly overwhelmingly most, but not all.
But if you say "well I don't believe in that or don't understand it enough to defend it at least" most trinitarians -- and certainly church hierarchies -- will NOT say "ok cool nobody understands God anyway so it's not a problem if you don't accept the Trinity"
No, that is NOT the reaction you will usually get UNLESS you are in a Progressive / Liberal Christian church. THEN you might get that relaxed, kind, accepting, cordial reaction. But the church does have to be really liberal for that. (or maybe Quaker, liberal or not, or of course, the explicitly non trinitarian churches, which are often very conservative and bible-literal in their own way)
Because in most places (regular churches) if you deny or even question the Trinity, you might get pretty unpleasant reactions, shocked faces, and snotty rude words from people.
And be unwelcome. Oh, and you'll DEFINITELY be told "You're not really Christian"
So -- the doctrine controversial because if you don't at least to pretend to agree with it, you are not well received.
It's controversial because it is defended too hard by its adherents.
It's controversial because classical Christian dogma doesn't allow you the freedom to simply speculate on the nature of God, or maintain a Jewish view of God. Nope, HAS to be the trinity. Too bad what you actually think or wonder.
It's controversial because it's one of those areas that classical Christianity has decided it has drawn a line in the sand and if you don't agree with it you've got a fight on your hands, and won't be able to remain in some churches.
What you said is right. Why would God make complete sense to us? Of course not!
The problem is, the Trinity doctrine at least sort of claims to have made sense of God, seems to be making truth claims, seems to be making factual claims about God, but if you're puzzled about it and show skepticism or if you want to adhere to the more fully "indivisible" idea of God found in Judaism or if you want to be free to speculate -- Well, you're seen as an apostate at the very least, and your ideas will be regarded at least in some circles as a threat to Christian teaching.
Somebody or many somebodies decided, very early on, that the trinity was FOUNDATIONAL to Christianity and no deviation was allowed, and many, many, MANY people still think that way.