Coming from someone who has been actively involved in studying Patristic Theology, Mystical Theology and other ancient Christian-related literature, the answer, from a CHRISTIAN, Biblical perspective, is: the same thing that happened.
The reasoning is simple: if you thumb through the Old Testament, you will find certain "prophets" that hint at the coming of the Messiah - the Messiah being the Theanthropo or God-Man, Jesus.
The problem, however, is not to be found in the Old Testament - seeing as Jewish are generally unanimous in their theological orientation. To answer your 'What IF..." question in a more succinct manner, I suggest you read (about) the early Christian literature - which contains HUNDREDS of documents that DO NOT agree with the general "orthodox" conception of Christianity. As a matter of fact, there is a Gospel of Judas wherein Judas and Jesus had PLANNED everything beforehand....
Pretty shocking, right?
All "Christians" that you'll stumble across have pretty much adopted the Nicene version of Christianity; that is: they adopted the Nicene Creed. There have ALWAYS existed and still DO exist different, ANCIENT, pre-Niece versions of Christianity: see Coptic Orthodox, Ethiopian Orthodox, Nestorian Church of the East, and etc. There were, at one point, Gnostic Christians, which, unfortunately, have been driven to extinction by "orthodox" Christianity.
Interesting stuff I tell you....