[Ali Imran]

it was in the Latin Vulgate and some later Latin manuscripts, as well as in certain Greek manuscripts... So this was not a forgery..

It was taken out because in the very early Greek manuscripts, it does not contain the phrase

does it affect or influence the core christian tenets..No

So the original author didn't write that. Someone added it much later, we don't know who. 

Why do you think they did that? 

My guess would be to silence the Christians who rejected the Trinity.