I would suggest that every false understanding of the Bible, every wrong doctrine, has some sort of psychological basis to it; and that often, this involves an excuse for flunking the challenge to believe God's word. To believe that Jesus of Nazareth was human, never sinned, died, and then rose again... demands a lot of faith. I recall discussing the nature of the Lord Jesus for many hours, late into the night in an apartment in South London. By about 2 a.m., we seem to have got to the crux of the issues. My friend said something to the effect, with a genuine sense of wonder, 'If you're asking me to believe that a man could live and never sin, die and then resurrect... I can't believe that of a man. I just... don't have the faith. I have to believe He was God to have done all that'. I left soon afterwards, and drove across the silent, sleeping suburbs of my hometown feeling that at last I had understood why there is so much belief in the Trinity, 'Jesus = God' idea. Quite simply, it demands much less faith. And to believe the simple Biblical account does actually require more faith than might at first appear. To believe that 2000 years ago, on a day in April, on a Friday afternoon, on a hill outside Jerusalem, a perfect man died... and after three days, the graveclothes stirred, a young man walked out into the early morning mist, with the lights of Jerusalem shimmering in the distance... that 40 days later He ascended up vertically into the sky and somehow got taken to Heaven, the very centre of the cosmos... yes, it demands faith to grasp the personal, actual, concrete, historical reality of it all. It's so much easier to shrug it all off, to walk away from the challenge of faith, by saying that yeah, actually, He was God. First century Israel stumbled at the humanity of Jesus. "Is not this the carpenter, the son of Mary, the brother of James? And they were offended at him" (Mk. 6:3). In essence, the same is happening to Trinitarians. They just can't hack that Jesus, Son of God, perfect human being... was truly human, with a human brother, mother and relatives. And so they have stumbled off into various wrong theories and theologies about Jesus to try to rationalize and spiritually legitimise their lack of faith in Him as a human person.
The early Christians must likewise have struggled with the questions- how could a man have done all this? How could this be true of a man? Could one of us really have pulled this off? And so they took the easy way out, flunked the issue, by deciding that Jesus must've been God. Likewise there is the challenge of the fact that Jesus is explained in Scripture as our representative; but that requires a lot of faith from us, and so Christianity generally has ditched that demand and replaced it with a pagan notion of substitution. Yet the Lord Jesus set us a pattern- humiliation and suffering, followed by glorification. Yet the common conception of Jesus gets this all the wrong way around- pre-existent glory in Heaven followed by humiliation, then a return to glory. But the Bible clearly teaches that the glory of the Lord Jesus was earned, it was His reward, and we with all our hearts say "Worthy is the lamb that was slain!" to receive that glory- knowing that we too have embarked upon a similar path to glory, with every experience of humiliation in this life understood in that context.
Despite the fact that Jesus evidently preferred to speak of Himself as "son of man", the disciples are never recorded as referring to Him in this way. This psychological discomfort with the human Jesus is reflected by the way in which the 2nd century Christian [heretic] Valentinus started teaching that Jesus ate and drunk "in a special manner, so that no excretal waste was produced" (1). Yet the Biblical emphasis is upon His eating and drinking as being a sign that He really was human, like us! This same strange discomfort with a real Christ continues to this day- there's always vociferous reaction against any Bible translation which has Jesus speaking in ordinary human language (e.g. that of Andy Gauss, The Unvarnished New Testament), and against any movie or piece of writing which shows the Lord Jesus experiencing the kinds of human feelings and passions which we do. The human desire to believe in a god rather than a man is demonstrated in Israel’s attitude to Moses. They complained about “this Moses, the man that brought us up out of the land of Egypt”; and therefore made the golden calf, proclaiming: “These be thy gods, O Israel, which brought thee up out of the land of Egypt” (Ex. 32:1,4). Note in passing how they created one calf, but worshipped it as gods plural. They committed the trinity fallacy of many centuries later. They couldn’t handle a saviour who was human, like them, and so they decided that a god had been their saviour, who existed as a plurality, gods, within a unity, i.e. the golden calf.
The essence of Christianity is to be as it were in a personality cult behind the person of the Lord Jesus. It's all about reflecting daily upon Him, asking "What would Jesus do?" as we face the myriad decisions which make up daily life. Yet this is hard to do; we find it almost impossible to maintain daily focus upon the Jesus who is revealed in the Gospels. The tendency always is to let our mind stray onto more abstract and less personally demanding things; and it has been observed that as the Church as a whole moved away from focus upon the real, human Jesus of the Gospels, so they became increasingly absorbed in speculation about His supposed previous life in Heaven.
We can see this discomfort with the literal and the real by the way in which Christians began to allegorize everything. To believe in the real Jesus, in the miracles of God in human lives over history, was too great a challenge to faith- and so everything was made comfortably abstract. The New Testament writers present things like the crossing of the Red Sea and the events in the wilderness as real historical events which were types of the work of Christ (1 Cor. 10:1-4; Hebrews 3 etc.). But by the second century, there was a shift away from reading these events as types, but rather they were seen as allegories- no longer were the events so importantly real, rather the characters and events were seen as allegorical (2). It was against this background of ever increasing abstraction that Christians likewise started to move away from the real Christ. Origen in the third century argued strongly that the historical sections of the Bible were to be taken as allegory and not as literally accurate history. He spoke of there being in the Bible "spiritual truth in historical falsehood", and went on to use this as an excuse to explain why the Lord Jesus is presented as human rather than Divine in the Gospels (3). And so, as so often, an incorrect base attitude to God's word led to seriously misunderstanding it.
Notes
(1) As quoted in Larry Hurtado, Lord Jesus Christ (Cambridge: Eerdmans, 2003) p. 528.
(2) This shift in exegetical style is documented at length in Martin Mulder & Harry Sysling, eds., Mikra: Text, Translation, Reading And Interpretation Of The Hebrew Bible In Ancient Judaism And Early Christianity (Assen: Van Gorcum, 1988) pp. 727-787.
(3) As quoted in Henry Chadwick, The Church In Ancient Society (Oxford: O.U.P., 2001) p. 135.