It's the Christian theology. Without it Christ could not be God and therefore Christianity couldn't exist. Any sect that denies trinity cannot call itself Christian.
The NT does mention Father,Son and the Holy Ghost. But it doesn't say it is one God, or that all three are co-eternal and co-equal. In fact, the NT calls on the Father God and the other two "lords" which is masters.
The biggest problem with Trinity is that is that you can't have three "Persons" in one Being who is simple and indivisible.
Also, different Christian sects and different Church Fathers speak of Trinity differently because the whole thing is incomprehensible. It's like saying 1 + 1 + 1 = 1.
The Nicene Creed, which is the Church's official statement of faith dioesn;t really speak of one God. It starts of by saying that we believe in ONE God, the Father Almighty, the maker of heaven and earth...so far so good.
But then it says we believe in ONE Lord (master) Jesus Christ who is a true God of true God [so obviously a different God!], who is of" one [same] essence (nature) with the Father...and that by him "all things were made, both visible and invisible" [I thought the Father was the maker of heave and earth!]
Then it says that (the Son) came down [sic] from heavens and was made flesh by (or from, of) the Holy Ghost and Virgin Mary and became man. And that he rose (himself) on the Third day [a change from the Pauline verse which says that God raised him] according to the scriptures, ascended to heavens and sits to the right of the Father...
And then it says we believe in the Holy Ghost, the Lord (master), the Giver of Life, who proceeds from the Father, who is glorified and worshiped together with the Father and the Son, who spoke through the prophets."
This does not describe one God or one master, but one God (Father), and two masters, who are obviously divine (one was born of the Father and the other one proceeds form the Father), but are never referred to as one and the same God as the Father. Obviously they are also not co-equal with each other or with the Father because one sits to the right of the Father and the other one doesn't...
Besides there is a contradiction in saying that we believe in ONE master, Jesus Christ and then to claim we ALSO believe in Holy Ghost, another master...
Might as well believe in pink unicorns on Jupiter.
Fair enough I guess, but if I remember you have an orthodox background. What happened that turned you from that path.
You don't have to answer that, it is just some what unusual in my epxerierence so it piqued my curiosity.
Correction
It’s the man version of Christian theology which is not doctrine!