John Shuck asked me in the comment section of the last post, if we can't know Jesus' authentic words by using form criticism, can we at least learn about earlier material?
My response, "Perhaps."
The reason for my ambiguous answer is that the application of form criticism and other assumptions are going to make this determination. Certainly form criticism identifies the literary type of the block of material (although this tells us next to nothing about its oral performance). If there are enough variants, it might even show us how earlier material becomes secondarily developed, such as variants placed in different literary contexts, reinterpreted with the addition of a secondary clause or phrase or introductory question, or developed into fictitious literary dialogues.
But once we leave these parameters, the going becomes rougher since ALL the materials we have about Jesus were remembered and written by and for the early Christians, including his parables and sayings. This means that the parables and sayings are no less "church material" than the miracle stories or apothegms or passion narrative. So trying to sort out some of this material as "less-churchy" than the rest is very problematic. The sayings of Jesus, especially the parables, are considered most authentic of the materials, as if they were preserved in his own diary untouched by the memories and needs of the early Christians.
Form critics seem to have realized this and so ventured to put into use the dissimilarity principle (and the principle of coherence) in order to determine which of the sayings were authentic and which represented the voice of the church. Jesus material is eliminated if there are parallels in early Judaism or early Christianity.
Of course this leads to a serious distortion of any historical Jesus recovered. And it is a way that the difficult apocalyptic materials have been removed from Jesus' mouth, even generating the argument that they are later additions made by the early Christians to the non-apocalyptic message of Jesus. Circular reasoning at its height.
What we end up with is a Jesus that doesn't look anything like anyone around him!
In my opinion, the application of this principle has been theologically-motivated from the start, and in some cases bordering on anti-Semitic. It allows the interpreter to control Jesus to the point that Jesus becomes a man against Judaism and other Jews around him, a man who has no self-consciousness as a Prophet or Messiah, and a man who is unlike all other first-century Jews. Jesus is unique.
This principle can tell us nothing about the historical Jesus, in fact it outright distorts him beyond recognition. It is a principle that we should never have applied in the way we have done. And it is time for it to go.