To declare Jesus to be the son of God! Are we not all made in the image of God? Jesus is more like a prophet or saint, and/or a Buddha! Or he should have been treated more like Abraham.
Monotheism as was then already common in Judaism and Zoroastrianism (but also in China and India) was a major advance in human civilization compared to then common polytheism (a la e.g. Greece, Egypt. Sometimes a different god for who knows what). Judaism also considered any worship of a human as idolatry. Another major advance!
The desire or urge to attach anthropomorphism to God was probably another motive to declare Jesus to be the son of God. This was misguided!
You may ask what difference does it make to treat Jesus either as a God or as a saint? How many humans in history have tried to be wannabe gods? It is also much more challenging to live up to the example of a son of God instead of a mere mortal prophet or saint! While Jews still believe in the coming of a messiah, Christians have noticed or realized that the attraction of Jesus more than 2,000 years later has been sort of fading. If a son of God can be borne once on earth, why not twice, why not a daughter of God?
The second biggest mistake was the exclusion or diminished role of women! Can you even imagine what a religion Christianity would have been with women as equals. E.g. peace on earth would have been much more likely!
P.S. This is not a condemnation or criticism of Christianity or any other religion, but some food for thought! No disrespect for the greatness of Jesus! I am not an atheist. Freedom of religion is extremely important! Most humans believe in something positive and that is good!
