In modern-day India, the Bhagavad Gita is probably the scripture that is most cited by modern Hindus. So if you really wanna understand the spiritual core of Hinduism, the best place to look are the Upanishads, and you also have the Bhagavad Gita, which even though it's part of the Mahabharata, which is considered Smriti, it is revered and is often considered to be divinely revealed by Hindus. Now, this isn't a comprehensive listing of all of the scripture that is important to Hindus. Obviously, you have the Vedas, which are very ancient. Even these epics, their events might have taken place 3,000 years ago or maybe more. Their composition was over 2,000 years ago. They were canonized during the Gupta Empire.