First of all, let’s apply sound historical method, that of biblical historians which is no different, so biblical historians assure us, from historical methods practiced by any other historians.
So to begin with, we will dispense with that cynical, hypersceptical, anti-supernaturalistic, post-Enlightenment hermeneutic of suspicion, and follow the dictates of the progressive, pre-Enlightenment (middle-dark age?), Christian ethic of the hermeneutic of charity. This means that if we read a statement by a fellow brother or sister then it is only a matter of civility at the very least to give his or her words the benefit of the doubt. That means that we can assume that the author of our text was, like ourselves of course, zealous to tell nothing but the truth, and to convey accurate historical information for the edification of their own and future generations.
Next, we will bring into play various criteria of authenticity as they may apply to our text in question.
In proof of this, as they say, they advance not legends, as the Greeks do, but manifest facts; for practically the entire inhabited world is their witness, in that it eagerly contributes to the honours of Isis because she manifests herself in healing. For standing above the sick in their sleep she gives them aid for their diseases and works remarkable cures upon such as submit themselves to her; and many who have been despaired of by their physicians because of the difficult nature of their malady are restored to health by her, while numbers who have altogether lost the use of their eyes or of some other part of their body, whenever they turn for help to this goddess, are restored to their previous condition.
Now how is a historian to respond to this testimony?
Note that here we have a historian appealing to “proof” and “manifest facts” as opposed to mere “legends”, and above all to “the entire inhabited world [as] their witness”! Obviously no historian could have written such words, and to have others preserve them until this very day, if there had been any attempt at exaggeration or outright falsehood. Obviously there were witnesses, or if you are hypersceptical, readers who were not witnesses who would obviously have called the author to account for such a statement unless it were known to be true!
No author could possibly have gotten away with such an astonishing claim unless his readership concurred with it. If he were really lying then his reputation would have been shot and his work would not have survived to be read by the next generation, let alone by us.
The plain historical fact is that thousands of witnesses were able to testify to the miraculous healing powers of the goddess Isis.
We can strengthen the historical authenticity of this record of the healings of Isis by applying a few criteria that biblical scholars assure us the stock in trade of all historians.
We have the criteria of multiple attestation. Isodorus, who wrote slightly earlier than Diodorus Siculus, composed hymns with the same sorts of testimonies. If Ishtar is to be identified with Isis, I have previously cited other independent testimonials to the healing miracles of Ishtar. (Do a search in the search box on the right column for Ishtar.)
As for the criteria that we must discount anything that is explicitly said to fulfil prophecy, I don’t know of any reason for disqualifying the Isis testimony on this account.
On the criteria of embarrassment? Well, it surely would have been embarrassing if the facts were otherwise, given the extent of the eyewitness support in the testimonial. I think it is safe to say that the fear of embarrassment kept the authors honest. Besides, a literate author like Diodorus Siculus no doubt had pretensions to be accepted among the sophisticated literati of his day, and to acknowledge belief in the miraculous prowess of a popular goddess was hardly likely to strengthen his ambitions in that direction. He obviously felt constrained by the facts of the evidence and eyewitness support that buttressed it.
And regarding the criterion of dissimilarity, even double dissimilarity — note that Diodorus Siculus himself observes that the reports are clearly “factual” and very unlike the “legends” of the Greeks. And as for any comparison with Jewish or Christian miracles, well, how many were able to claim that Jesus’ miracles were known throughout the entire world through eyewitnesses. The miraculous performances of the goddess clearly pass the criterion of double dissimilarity test.
So there you have it, folks. Whom will you choose? If Jesus be a healer (not forgetting his record in his own home town, or his clumsy attempts to have a double swipe with spittle on the blind man) then follow him; but if Isis has the runs on the board, then you’d be mad not to back her, I reckon.