So to start off with, I can’t find the sources. I watched a “free quran education” video on isa AS and I saw a couple of reddit posts saying that in the torah that sulayman AS is the son of God metaphorically(so son of God just means he is very important, not that he is literally the son of God) and that banu Israel is also metaphorically the children of God.
Also if there is an absolute true answer answer then tell me in the comments please
OK so my theory:
The injil said something along the lines of “Jesus is the son of God”. I think this was also metaphorical because, well… Allah has no children.
Then, after Isa (AS)’s ascention, people preached to the polytheists (the Greeks, Egyptians etc.) and told them Isa (AS) looked like one of their existing gods(So they portrayed Isa (AS) as a Greek god to the Greeks and a Roman god to the Romans etc.)
The polytheists read the bible and presume he is literally the son of God due to them thinking Isa (as) looks like a god of theirs. Because The Romans were the centre of the christians world, they were the main producers of the bible, and because of that, then they corrupted the majority of the Christian ummah, because of them being misinformed.
As for the Christians who had the original Injil (if there were any), they were probably scraped off of christian history along with all other Christians in the middle east due to them doing different things than the Europeans.