It seems that in the first century Roman Empire the use of divine metaphors for humans was not uncommon. In the case of Jesus Christ the Church took these metaphors too seriously and literally.
Prof. Bart Ehrman says:
“Apollonius lived at about the same time as Jesus, although they never knew each other. Their followers, though, knew each other and had heated debates about who was superior.
These were not the only two men believed to be divine. Jesus may be the only miracle-working Son of God that we know about in our world, but he was not at all the only one talked about in his world.”
(Prof. Bart Ehrman. The Historical Jesus. The Teaching Company course guidebook, 2000. Page 9.)
Polytheism was in the air as the Christianity was being formulated under Paul’s directions, many of us do not appreciate that Judaism was a small minority in the Roman Empire, forming perhaps 7% of the population. Majority of the Empire was polytheist and it was in those circumstances that Paul was trying to create a politically correct monotheistic religion and we were given ‘three in one’ rather than multiple gods, a vast improvement towards monotheism!
What caused the monotheism of Judaism be distorted into Trinity can perhaps be imagined from a few verses in Genesis, which show the polytheistic residuals even in the monotheistic tradition of the Old Testament and Judaism:
When human beings began to increase in number on the earth and daughters were born to them, the sons of God saw that the daughters of humans were beautiful, and they married any of them they chose. Then the LORD said, “My Spirit will not contend with humans forever, for they are mortal; their days will be a hundred and twenty years.”
The Nephilim were on the earth in those days—and also afterward—when the sons of God went to the daughters of humans and had children by them. They were the heroes of old, men of renown.