Upanishades and Brahamanas
The unique character of ancient Indian civilization centered on the institution of caste and the doctrine of reincarnation, or rebirth. The search for freedom from the wheel of rebirth led to a variety of types of piety.
The oldest religious texts in India are the Vedas. The Vedas consists of songs recited during sacrifices, and passages instructing priests, or Brahmans, what to do during the ceremony.
As time went on some Vedas became less intelligible and performing rituals correctly became more important, since a single mispronounced word might provoke divine displeasure.
This enhanced the power of the Brahmans who insisted on proper invocation, and some Brahmans claimed that they could compel the gods to grant what was asked of them. Such extravagent priestly claims were put forward in texts called Brahamanas, a sort of later commentaries on the Vedas.
Not everybody agreed with this approach and a rival type of salvation was proposed in a literature called Upanishades, which suggested withdrawal and meditation for holy men, whereby a release from the sufferings of existence could be obtained without obedience to the priests.
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Hymns from the Rig Veda
A comprehensive introduction to the Upanishades