"The Upanishads" represent for the Hindu approximately what the New Testament represents for the Christian. The earliest of these spiritual treatises, which vary greatly in length, were put down in Sanskrit between 800 and 400 BC.
The Upanishads, the earliest of which were composed in Sanskrit between 800 and 400 bce by sages and poets, form part of the Vedas - the sacred and ancient scriptures that are the basis of the Hindu religion. Each Upanishad, or lesson, takes up a theme ranging from the attainment of spiritual bliss to karma and rebirth, and collectively they are meditations on life, death and immortality. The essence of their teachings is that truth can by reached by faith rather than by thought, and that the spirit of God is within each of us - we need not fear death as we carry within us the promise of eternal life.