Sometimes they even argue that Jesus influenced the progress of history through Old Testament times. They do this when it seems necessary to rationalise the inconsistencies in the flow of history. They try to make it seem that things defying the normal concepts of cause and effect are still consistent with some sort of truth. I feel that this is intellectually dishonest and logically parsimonious, but let us set that aside for now.
Let us think about the question of the origin of the Trinity. Where did this concept come from? It sounds like a silly and trivial question. Surely it is in the bible. Or is it?
The Old Testament certainly seems not to mention the idea that God is in three parts. It is not that it doesn't mention the host of heaven, a possible wife and concepts of a hierarchy of gods in heaven. These ideas are found in profusion, especially in the early books. But God himself never reveals to his chosen people that he has a split personality. His son never gets a mention and the Holy Spirit is only present in other vague terminology, such as the will of God. Neither of God's alter-egos really gets a look in for many centuries, and within Judaism nothing has changed to this very day.
If he really is a trinitarian God, wouldn't you think that he might have let something slip to his chosen people to that effect?
So presumably the New Testament reveals something about it? Well - you might expect this, but it is generally held that there is only one verse that might refer to the trinity. This verse comes in one of the lesser books which has a somewhat unknown provenance. The First Epistle of John might have been written by someone called John, but it is very unlikely to have been one of the significant Johns. In this letter, there is one verse which says (in one translation):
There are three in heaven that bear witness . . . these three are one. -- 1 John 5: 7
But when you look at this in context, (and another translation) what does it mean? Not much, I suggest!
7 For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one.
8 And there are three that bear witness in earth, the Spirit, and the water, and the blood: and these three agree in one.
9 If we receive the witness of men, the witness of God is greater: for this is the witness of God which he hath testified of his Son.
Now try to tell me that this is the best way to reveal the surprising fact that God is in three parts, and you will have trouble convincing me. Something as important as this needs to be set out in much clearer words, not in flowery metaphor in an obscure letter written by an unknown author.
The apostle John opens his account of the life of Jesus Christ with this declaration: "In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God. All things were made through Him, and without Him nothing was made that was made . . ."
So that is clear too? Beautiful prose, full of rhetorical devices, but it makes no sense unless you chose to claim that "the word" means Jesus.
Theophilus of Antioch, 170 CE first used the Greek word for Trinity. Over a century later, in 325, the First Council of Nicaea established the doctrine of the Trinity as orthodoxy and adopted the Nicene Creed, which described Christ as "God of God, Light of Light, very God of very God, begotten, not made, being of one substance with the Father".
This isn't a biblical truth, it is just 4th Century Athanasian doctrine that happened to prevail over another point of view - namely that of Arius!
Arius held that the Father, Son, and Holy Spirit were three separate essences or substances and that the Son and Spirit derived their divinity from the Father, were created in time, and were inferior to the Godhead. Arianism was largely condemned to the annals of history at the Council of Nicaea (325), although a few sects still follow this point of view, including the Jehovah's Witnesses.
So, having started out with the assumption that the whole business of the Trinity was easy to interpret, I conclude that there is no end to the lengths theologians will go to to 'prove' a point, and that there is little to suggest a biblical basis. The trinity was not invented by God at all, not explained by God in the bible and in fact it is a logically dubious invention of men.
Since I have grave doubts about the actual existence of an historical Jesus, due to the paucity of non-biblical evidence for any of the history of his life, I wonder why anyone would waste so much of their lives inventing doctrines like this.
Indeed, why do I waste my time even thinking about it?
The answer for me is related to a search for truth. For theologians I doubt they have such pure motives.