Upanishads, Brahma Sutra, Bhagwad Gita are the basis of Vedanta Philosophy..
but today these so called spritual leaders or I would say JOKERS, neglect them and I become astonished when they piggy-back Adi Shankara to justify their whimsical mental speculation.
Even the philosophy of Advaita has its basis from Bhagwad Gita and other Vedic literature.
Jai Prabhupada!