Barnett opens in an Aryan village of the eastern Panjab some thirty centuries ago, and asks what its people believed. He finds two religions living together. The common folk care about charms and magic and revere certain old heroes as helpful spirits. The priests, the brahmans, chant elaborate hymns, the Rig-veda, to many nature gods such as the Sky-father, the Dawn, Agni the fire, and Soma. The priests already half-believe that the rite itself, performed with exactness, has a magic power over nature. Barnett asks the reader to mark this idea, because much will issue from it.
In the age of the Brahmanas the tribes have become kingdoms ruled by kings and, spiritually, by an established Brahman priesthood. That priesthood works the logic of ritual to its end and builds perhaps the most colossal sacrificial system the world has seen. The strange result is that the system grows practically godless: the universe is run by an impersonal cosmic principle and by the soulless force of the rite, and God is no longer the highest power. Barnett calls this, in a careful sense, the atheism of the Brahmans.
A reaction follows within the priesthood itself. The thinkers of the Upanishads decide that works matter only as a path to knowledge, and that once knowledge is gained, works fall away. The knowledge they seek is of Brahma, the single, infinite, impersonal Reality that is both Being and Thought. The supreme secret is to realise that one's own soul is identical with Brahma. Alongside this comes the doctrine of karma and samsara: each act earns its reward across an endless chain of rebirths, and only the saving knowledge of Brahma releases the soul from the wheel.
The third age is the age of the epics and the gods who rise to the front, Vishnu and Siva. In the Mahabharata, Vishnu, once a faint spirit of the sacrifice, becomes a gracious saviour who incarnates himself to restore righteousness, and is identified with the hero Krishna. The Bhagavad-gita, spoken by Krishna to Arjuna on the eve of battle, teaches that duty done without selfish motive, and devotion offered to the Lord, lead to salvation as surely as the old path of knowledge. Caste is kept, but caste-duty done in a spirit of sacrifice becomes a road to God open to all. The Ramayana tells the parallel story of Rama, a human hero of Ayodhya who is raised in the later books into an incarnation of Vishnu.
Barnett closes by asking whether any single principle runs through this bewildering variety. He answers that the history of a god is shaped by two forces: the deepening spiritual experience of a people, and the character of its religious teachers. India in particular tends to read the guru as a manifestation of the god, so each strong teacher stamps something of his own mind on the deity he preaches. Gods are thus recast again and again in the mould of human hopes and ideals, which is why, as an old priestly saying puts it, in the beginning the gods were mortal.