Without having read the 'a fall away novel', it's hard to determine the main theme. But if we break down the phrase, 'fall away' might suggest themes of loss, separation, or the decline of something. It could be a relationship, a belief system, or a character's self - worth within the story.
The main theme of this 'a fall away novel' remains a mystery without more details. However, generally, the term 'fall away' can imply a sense of moving away from something positive. So it could potentially explore themes like disillusionment, the end of an era, or the departure from a certain path in life. Maybe it's about a character who was once on a good track but then things started to go wrong and they 'fell away' from that initial state.
Since I don't know the exact 'a fall away novel', it's impossible to say for sure. It could be about a character's fall from grace, perhaps in a social or moral sense.