I haven't cracked it either after lengthy staring, but here are my thoughts for what they're worth.
Unless it's a big red herring, the title might suggest the Book of Revelations written by John. If so, 15 across could be the entry to highlight. To get 39 cells, an odd number, on an axis of symmetry surely they must cross one cell of the entry word. I don't see how you can get an odd number of cells symmetrically disposed otherwise. A vaguely circular arrangement of cells crossing one of the cells of the entry word (which surely must be centrally placed) could total 39.
However, none of the above has led me anywhere, so it may be completely wrong.
Did anyone else think the last word in 15 is a dubious definition for that element in the wordplay? I couldn't find any support in Chambers.