Jesus, Moses, Abraham, Isaac, Jacob, Mathew, Mark, Luke, John, all those prophets - there were as historically real as Alexander the Great, King Tut, or any other figure in ancient history. Nothing hypothetical about them.
And that, in a nutshell, is the crux of the argument. Christianity is based on historical figures involved in historical events in real space and time. It is not based on philosophical speculation, nor on fantasy, but on history.