"Paul refers to the 'mystery' of the gospel that was 'hidden' from rulers of this age and of the 'wisdom, secret and hidden,' that was given only to those who were 'mature' (1 Cor. 2:6–7). Is this not a reference to secret knowledge given only to a few in the church, who could see below the surface of Paul's words and learn the hidden knowledge that could bring liberation?" [Bart Ehrman, Peter, Paul and Mary Magdalene p. 161]
But the Marcionites had the original understanding of this passage and they thought thought the same apostle revealed the first written gospel. Surely, they thought that he was referencing a 'secret gospel' rather than an esoteric teaching passed by word of mouth. For some reason, this alleged 'expert' on the Marcionites never makes the connection to what MUST HAVE BEEN the sectarian interpretation of this passage. Von Harnack would have figured it out - but he didn't have the luxury of Morton Smith's discovery.
Oh yes. Ehrman suspects that the text is a forgery. Maybe it's because he is frustrated by his own inabilities to figure out its historical context.
Stephan Huller has had a life long interest in religion and spirituality 
dating back to conversations with his grandfather, Gaston Frank. "He said we represent one of the last descendants of the Frankist Jewish faith in the world," he muses. "I grew up thinking that our family was something like the Last of the Mohicans."