Actually Shankara thought the Gita was in fact the epitome of the essentials of all Vedic teaching. Madhva, commenting on the Vedanta-sutras (2.1.6), quotes the Bhavisya Purana, which states, "The Rig-veda, Yajur-veda, Sama-veda, Atharva-veda, Mahabharata, Pancharatra, and the original Ramayana are all considered Vedic literature. The Vaishnava supplements, the Puranas, are also Vedic literature."
The Chandogya Upanisad (7.1.4) mentions the Puranas and Itihasas as the fifth Veda. The Srimad-Bhagavatam (1.4.20) also states, "The four divisions of the original sources of knowledge [the Vedas] were made separately. But the historical facts and authentic stories mentioned in the Puranas are called the fifth Veda."
Therefore, the Vedas themselves not only accept the four Vedas, the Upanishads, and Vedanta-sutras, but also the Mahabharata, Bhagavad Gita, the Ramayana, and the Puranas as being authentic Vedic literature.
The point is, to be accepted as Vedic literature it must present the same purpose as the original texts. But if it deviates from the Vedic conclusion or is a hodgepodge of various concocted philosophies, as are many viewpoints that one will find merged under the name of "Hinduism," then it cannot be relied upon.
Therefore, to be sure something is authorized, we only accept the established Vedic teachings that are supported in the many Vedic texts. So Buddhism, Jainism and Sikhism, which do not refer to or support the conclusions of the Vedic texts, are considered non-Vedic, although outgrowths of Vedic philosophy and accepted as part of Hinduism by some.
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