1928. The lineal key to the Great School of the Masters. Volume 5 of the Harmonic Series. The Great Message is the lineal key to the relationships between all past religious and philosophical world movements and the Great School of the Masters. The Great School was or is behind or is in sympathy with every great humanitarian movement known to history, and the Great School, throughout all recorded ages, has been and still is a physical association of men who have attained mastership.