There was only one requirement to the Unity Technique of Great Educational Heavenly Devil Scriptures, which was to unite all the spells, divine arts, sword skills and battle techniques in Great Educational Heavenly Devil Scriptures which included the Seven Writings of Creation.
When one managed to do this, they could then be considered unified.
When the previous cult masters received the teachings from the woodcutter on the rock, they would all have to comprehend their own Unity Technique and only when they succeeded then could they secure their position of cult master.
Otherwise for sects like Heavenly Devil Cult which didn't mind their cult master dying, changing the cult master would become a common thing.
