In the above quote by Arnold he says this:
"I know of no one fact in the history of mankind which is proved by better and fuller evidence of every sort, to the understanding of a fair inquirer, than the great sign which God hath given us that Christ died and rose again from the dead."
I'm just postulating here but I would have to believe that part of the reason for his belief would be the following I mentioned in another post:
Here is a little info about the New Testament manuscripts in existence:
From the article: Why I Believe The New Testament Is Historically Reliable by Gary Habermas
"...significant portions of some ancient works are missing. For example, 107 of Livy's 142 books of Roman history have been lost. Of Tacitus's original Histories and Annals, only approximately half remain.
The fact that there is outstanding manuscript evidence for the New Testament documents is even admitted by critical scholars.2 John A.T. Robinson succinctly explains, "The wealth of manuscripts, and above all the narrow interval of time between the writing and the earliest extant copies,
make it by far the best attested text of any ancient writing in the world...
...The result of all this is an incredibly accurate New Testament text. John Wenham asks why it is that, in spite of the "great diversity" in our copies, the texts are still relativity homogeneous. He responds, "The only satisfactory answer seems to be that its homogeneity stems from an exceedingly early text-virtually, that is, from the autographs."5
The resulting text is 99.99 percent accurate, and the remaining questions do not affect any area of cardinal Christian doctrine."
http://www.apologetics.com/default.j...bermas-nt.html
Also there are over 5000 manuscripts of the original Gospels whereas the average number is 8-10 manuscripts for other ancient writings.