I'm not sure if there is a specific thread on Secret Mark - feel free to start one if you wish.
As for the Jesus Seminar - I'm not at all sure how they claim to be able to make probability statements as to whether an historical Jesus made any single statement. Although there is room to make literary criticism based on surviving texts and fragments from within the first couple of centuries, the supposed thoroughness of the Jesus Seminar itself I view with strong scepticism. Essentially, we seem to have a crew of self-appointed experts making judgements on an apparently rather arbirary basis.
I stand to be corrected, of course.
