jno, OK. You asked 'At what point does a 'Jew' who believes the messiah has arrived become a 'Christian'?', and tell me this is related to sigma's original question so you are clearly referring to Jews living in the time of Jesus. I can only say that Christianity as we would recognise it didn't exist at that time. The people who believed that Jesus was the Messiah remained Jews and continued to adhere to Jewish law.
I don't believe anyone could in all honesty accuse me of being afraid to meet a challenge, but as far as evidence of alterations in ancient texts are concerned, it's a big subject which I don't have time to go into here in great detail. However, since you are so insistent, and since your interest is clearly not sufficient to entice you to research the matter for yourself, here is a page written by Graham Stanton, Professor of New Testament Studies at the University of London. It may not be the best link available, but it might suffice for your purposes.
http://www.bowness.demon.co.uk/reli2.htm
You say being edited doesn't necessarily make the Gospels untrue, but I beg to differ. If elements of a story are proven to have been altered, then we cannot be confident that other elements have not also been revised, and that must cast doubt on the veracity of the story as a whole.
cont....