"Do you think some Christians somewhere just made up the whole doctrine..."
>>> That's exactly what happened and rather late in the game.
"It was taught from the very start of Christianity and has always been believed."
>>> Not so. From my earlier post:
Bear in mind, there is no mystery as to the origins of the Trinity, nor is there any secret for how this aberrant doctrine emerged. The doctrine of the Trinity was forged out of the crucible of the Catholic Church long after the Christian century. It is, therefore, no wonder that this pagan doctrine was unknown to authors of the New Testament. Church history reveals that it was not until three hundred years after the birth of Christianity that the doctrine of the Bianity (325 C.E.) and Trinity (381 C.E.) received formal approval in the Christian community. These well documented events occurred under circumstances rife with contention, political agitation, and radical dissension in the early Church.
The trinity is entirely MAN made, and not from God. You shall have no other gods before me - That's from God. That the Council of Nicea in the fourth century formally defined the doctrine of the Trinity, does not mean it didn't exists until then. The Deity of Jesus Christ was established by Jesus himself, it was taught in the Old Testament Scriptures and expounded upon in the writings that make up the New Testament. There is no reason why Christians would have invented such a doctrine centuries later about such a critical point of the very nature of Jesus Christ! The Nicean Council was assembled to dispute the heresies propagated by false teachers like the Gnostics and those who rejected that Jesus even HAD a physical body. Their lies were exposed as false by the writings of the Apostles who wrote as they were moved by the Holy Spirit.
You read these verses here: http://www.kingjamesbibleonline.org/Bible-Verses-About-The-Trinity/