The danger is of a fall into psychism, mediumship, sorcery, and black magic--above all the danger of stimulating the personal ego--which accompanies the abuse and misuse of mystical knowledge by those unready or unworthy of it. It was awareness of these dangers both by the official heads of certain religions and by its solitary adepts which kept mysticism a secret hidden and guarded from the public for centuries and left them with the relatively harmless dogmas and theatrical parades of public religion. But continued silence would have been even worse than these evils while the waves of materialistic belief washed over humanity. Because humanity has been losing its religious faith and growing worse in its moral character, even though it has been gaining in technical skill and scientific knowledge, much knowledge has been given out that was formerly kept esoteric. The practical teachings about meditation especially have been given out for the benefit of those intuitive enough to heed them.
-- Notebooks Category 20: What Is Philosophy? > Chapter 2: Its Contemporary Influence > # 245