In the beginning of The Origin of the New Testament (1925, bibliography), Adolph von Harnack asks five questions which he says pose “at least five great historical problems.”
The five problems are these:
1. What is the reason and how did it come about that a second authoritative collection of books arose among Christians? Why were they not satisfied with the Old Testament, or with a Christian edition of the Old Testament, or–if they must needs have a new collection –why did they not reject the old? Why did they take upon themselves the burden and complication of two collections? Finally, when did the idea of a fixed second collection first appear?
2. Why does the New Testament contain other works in addition to the Gospels, and thus appear as a whole with two divisions (Gospel and Apostle)?
3. Why does the New Testament contain four gospels and not one only?
4. Why could only one “Revelation” keep its place in the New Testament? Why not several or none at all?
5. Was the New Testament created consciously? How did the Churches arrive at one common New Testament, seeing that the individual communities, or provincial Churches, were independent, and that the Church was one only in idea?
Harnack then follows immediately with:
From the standpoint of the Apostolic Epoch these five questions appear as just so many enormous paradoxes so long as one does not go deeply beneath the surface of events as they developed. I purpose to attempt a brief discussion of these questions; and it would be perhaps much to the point if future works on the history of the Canon of the New Testament started this way.
I have not yet found evidence that scholars of the New Testament canon have heeded his suggestion.