Grafton relies on new evidence taken from Smith’s recently published correspondence with his life-long friend, the great Jewish scholar Gershom Scholem. These letters make it clear beyond any reasonable doubt that Smith could not have forged either Clement’s letter or the passages of “Secret Mark” contained therein.Actually, if you go to the link and read Grafton's essay on Stroumsa's book, you will see that Grafton doesn't quite say he thinks the case is made beyond reasonable doubt.
Myself, I don't have a view either way on the genuineness of Clement's letter. I am unpersuaded by the efforts to decide the question by psychologizing what Morton Smith could or could not have done. Mind reading is an unreliable way to find the truth. In the absence of compelling evidence, I am inclined to give Smith the benefit of the doubt.
The only way to determine whether the letter is a twentieth-century forgery is to find the manuscript again (or perhaps another manuscript) and do materials tests on it to determine when the letter was copied into it. That might work. Or it might not.
And even if the letter itself is genuine, that doesn't mean that it quotes from an early, let alone the earliest, version of the Gospel of Mark.
But I haven't put much effort into Secret Mark. You should pay more attention to the views of people who have. Tony Burke held a symposium on the subject in 2011, but all the links in past PaleoJudaica posts seem to go to dead ends in an old version of his blog. But there is some information here. See also here and here. And if you poke around in the archive of the current version of his Apocryphicity Blog, you will probably find more.
Cross-file under New Testament Apocrypha Watch.
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