Who is Robert Crossfield/Prophet Onias from Under the Banner of Heaven: A Story of Violent Faith and what is their importance?

Robert Crossfield is the writer of the Second Book of Commandments. As a result of this book Crossfield is excommunicated from both the Mormon and the Fundamentalist Mormon Churches. It is Crossfield who heads the School of the Prophets of which Ron and Dan are members. It is under Crossfield's leadership that Ron receives the removal revelation.