Other concepts and words accepted in most of Christianity that do not appear in the bible:

1) The Rapture
2) Purgatory
3) The Seven Deadly Sins
4) The Immaculate Conception as it refers to Mary the mother of Jesus
5) “God helps those who help themselves”

So, what I hear you saying is that you DO believe in the Father, Son, and Holy Spirit, (all of whom are biblical, as in they are mentioned in the Bible) and also accept the “non-biblical” theory that somehow they are three persons in one -the “Trinity"- , yet at the same time, you’re wondering why the Trinity is so important. So, I’m just curious… if you’re willing to accept the Trinity, why does the importance of it interest you? Do you think the theory of the Trinity is important? Why or why not?

My biggest concern is that if this cornerstone of Christian theology can slip in, what else do we just accept without considering the source?  Like some of the items in the list that Brook posted, what are we taking for granted that isn’t there?

More than anything else, I wonder why a theory like this is such an in or out clause for many Christian churches.

Hi guys!
I found your blog through a link on an atheist blog which i found from a link on atheistgirls...a blog i like reading every onceinawhile because i think it helps keep me on my toes.  I thought your site (and your post) sounded as though they came from the heart of a sincere believer rather than someone who was interested in posting for argument’s sake.  I was right. I’ve enjoyed poking around and will probably stay on your site for a bit. 
I wanted to comment on this post, because i have had some of the same thoughts.  I don’t know if you’ve taken any theology courses, but i just started going to Bible school, and this sort of thing comes up fairly often.
The doctrine is a result of looking at the Bible and trying to come up with the best answer--the doctrine of the Trinity is in the Bible, but that the particular term is never used.  It came about as a result of people misreading or misinterpreting the Bible, when Christianity was a new idea.  Councils established canon and doctrine not so much to create dogma as to refute ideas which were not truly biblical.  For example, a church might have had the teaching that Christ replaced the God of the old testament, or that Christ was the son and thus a lesser God, or that the Holy spirit was not equally God, etc.  However, non of these ideas are biblical, so the term trinity thus defined the biblical standard.  In essence, the trinity was not a new doctrine or extra-biblical, it was just a clarification as a response to explain non-biblical ideas, show how other theories were not accurate, and the term is new.  That God is three-in-One has no verse, but can be deduced by looking at the scriptures.
It’s a non-negotiable for many churches because it differentiates from those that would say they believe in only one God and end there (because these usually don’t accept Christ’s divinity) or a belief that God, Christ, and the Holy Spirit are three separate gods, or deny the existence of the Holy Spirit as actual Person (as opposed to a feeling or force).
As for brook’s list, 2,3, and 5 are not biblical, nor doctrine (in evangelical/protestant churches), and 1 and 5 are only different terms for biblical teachings--the Bible says that believers will be “caught up” and clearly states that Mary was a virgin at the time of Christ’s birth, so immaculate in the sense that He was conceived without sin. 
The idea of our “sin nature” would also fall under this category, but again, i think it’s just the result of lots of debate about what the Bible says and boiled down and crystallized into a short simple term that conveys all the biblical support behind it rather than having to re-establish christian beliefs every time one starts a discussion.

Sorry for such a long post, hope it made sense.  I’m not an expert, just learning, and enjoying the search.

Comment:
“That God is three-in-One has no verse, but can be deduced by looking at the scriptures”
Response:
1 John 5:7 (King James Version)
For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one.