What about Vedas and Upanishads
Now the question is how they survived for so long?
The answer is they had passed through ages via oral transmission by each generation of people memorizing them.
Earlier parts of Vedas are called Samhitas and the commentary on them is called Brahmanas.
The combination of Samhitas and Brahmanas is called as Karma-Kanda.
Karma Kanda serves the purpose of leading and celebrating life with guided ceremonies and prayers.
Aranyakas and Upanishads are the knowledge parts of Vedas.
The combination of Aranyakas and Upanishads is called as Jnana kanda.
Why they are not so popular outside India and even in India.
The Muslim rule, Colonization of India by the British, ruined the Sanskrit language in India, and India lost its originality partially,
However recently a lot of activity happening by various Teachers and Gurus to bring back the glory of Vedas and Upanishads by publishing videos and teachings.
Please note that the purpose of Vedas and Upanishads is to have a happy living with harmony and attain Moksha.
They don’t promise Heaven or Hell, which we already experience in this lifetime because of our Karma. (as per Vedas not preached by me)
Here below is one such institute which recently started publishing good information.