From Karen Armstrong’s A History of God: The 4,000-Year Quest of Judaism, Christianity and Islam:

Not everybody was capable of philosophical thought, however, so Falsafah was only for an intellectual elite.  It would confuse the masses and lead them into an error that imperiled their eternal salvation.  Hence the importance of the esoteric tradition, which kept these dangerous doctrines from those unfitted to receive them.  It was just the same with Sufism and the batini studies of the Ismalis; if unsuitable people attempted these mental disciplines they could become seriously ill and develop all kinds of psychological disorders.