I've always heard and believed that while we don't know exactly who wrote them, and that they contain some facts, there is a sort of consensus between scholars that the traditional views on authorship are wrong, yet when I told someone this in a debate (granted it was at a church) they said I needed to read more about the topic and most scholars believe the traditional views.
So I'd like to know what are the main views by historians on there authorship? (Obviously there will be many)
Edit: Also is there any truth in the claim of 2/3 of the Roman Empire were slaves? It just seems a really high number, but I don't know much about the Roman Empire so it could be fact for all I know.
You might get some better answers for this question in /r/academicbiblical, particularly on the question of how many scholars believe the traditional views.
However, in terms of alternative views, there really aren't any. Matthew was either written by Matthew, or by someone lost to history: I don't think anyone has made a serious case that it was written by someone else we've heard of. The same for the other Gospels. There's some suggestions about the type of person who might have written each one, based on the text: see the introduction to any decent commentary for details.
The only slight exception is Mark. Here the (or at least a) traditional view is that John Mark wrote it under the direction of Peter, and this fits the content, so it's at least possible that Peter wrote it and John Mark had nothing to do with it.
First off, this is rather pathetic reasoning on part of your discussion partner, which amounts to "read more authors that agree with me and you'll see I'm right."
Secondly, you might want to elaborate what you mean by the "consensus view" or the "traditional views", because those terms could mean pretty much anything to different people.