Sanskrit is the sacred language of Hinduism, the language of classical Hindu philosophy, and the language of many later Buddhist and Jain texts.

The Upanishads are Sanskrit texts of the late Vedic and post-Vedic periods that mark the shift from the ritual focus of the early Vedas to philosophical questions about the self and ultimate reality. We see the emergence of Hinduism’s central religious concepts: Brahman (ultimate reality), Atman (the self), karma, rebirth, and moksha (liberation).

The Bhagavad Gita is part of the Mahabharata and a synthesis of various strands of Indian religious thought, including the Vedic concept of dharma (duty, rightful action); Sankhya-based yoga and jñana (knowledge); and Bhakti (devotion).

The Dhammapada, written in Pali, is a collection of sayings of the Buddha in verse form and one of the most widely read and best-known Buddhist scriptures. The Buddha was a wandering religious teacher who lived in the eastern Indo-Gangetic Plains around the 5th century BCE. 

You could start here:

The Upanishads, c. 800 – 300 BCE

The Bhagavad Gita, c. 400 BCE – 200 CE

The Dhammapada, 3rd century BCE and onward

Below is a map of the spread of Buddhism, Confucian teachings, and Daoism in the 3rd century CE. The Dhammapada was an important Buddhist wisdom text, and a Chinese translation appeared in the 3rd century CE.