The poem of the Bhagavad Gita is part of the longer epic, the Mahabharata, but it could be an insertion and in essence an Upanishad. It assumes Upanished like knowledge. The Upanishads have the authority of mystical experience and the direct authority of God or revelation.
The Bhagavad Gita can be seen as preventing the spread of Buddhism to the extent of consuming Hindu beliefs. Hindus had this resource to draw upon from over 3000 years ago (with the rise of Buddhism over 2500 years ago - these quantities of years are imprecise). Buddhism was a protest against the priestcraft and elitism of rituals around the earlier Hindu texts, such as the Rig Veda. Just as Buddhism did, the Gita makes itself accessible to all. It meets some of the challenges towards renewal the reform into a new religion, Buddhism, also met. The Gita, like Buddhism, is about one's dharma or path through life and how to achieve spiritual and human maturity. It is a response, if not wholly successful, to religious elitism.
Arjuna is worried about his path through life, in the context of two armies which are related to each other. He calls on Krishna his charioteer to help him through his dilemmas, or loss of confidence. Krishna speaks with Arjuna for some seven hundred verses before relaying a mystical experience of being God revealed.
The Gita if read in a surface fashion can be seen as an invitation to carry out war with full conviction. However, it also shows a condition of unstoppable war, which serves as a warning. Some of the imagery of the Gita, the power and light by which Krishna reveals himself as the incarnation of God, consuming all, is for us a vision of destruction in all an consuming war.
Gandhi saw the war setting as an allegory. It is a war within, or an intense setting of sharply divided relatives for moral and spiritual choices. He rather interpreted, unlike the nationalist R. G. Tilak before him (from 1897), the Gita as truth-force (Satyaghraha) which was peaceful. Gandhi favoured non-violent resistance, although he was killed by a Hindu nationalist himself interpreting the Gita as an act that Arjuna would have done. But Krishna tells Arjuna to be compassionate to friend and enemy, and see himself in everyone, and to take on others' sufferings. God is in everyone and everywhere. Although Arjuna goes on to fight with strength and wins the war, almost everyone is dead.
Gandhi had been too pro-Muslim, but the Gita has Krishna saying that even someone who worships incorrectly in the end worships him. This is the pluralism at the heart of the Gita. Gandhi himself enjoyed the first twenty verses of the Bhagavad Gita because it shows Krishna as disinterested, rather like we understand the advanced spiritual state of the Buddha.
The Gita proposes that we proceed through life spiritually through yoga. Yoga is the method through the dharma. Yoga can be meditation of the mind, action in what we do fully, or devotion in terms of worship. You can do one, or many. Again it is part of the plurality at the heart of the book, just as is tolerance of different ways of belief and faith. Hindu once was a label of a geographical people rather than a faith.
The Bhagavad Gita had become a nationalist's book, whether via Tilak or Gandhi. Before this it had also become (though it was not beforehand) a kind of substitute bible for Hindus. It became this because of the influence of Christian missionaries as well as the continuing place of the Koran for Muslims. The Gita became a book to learn in Sanskrit, for devotees and pupils, whereas no one spoke Sanskrit as such and relied on translations. The Gita is not a mythic history-theology, like the Bible, not commands, like the Koran, but is a book of choices towards moral and spiritual improvement and how one choice leads to another.
So the Gita can be read in a surface way, but it has depths of meaning for the person on a spiritual journey. Unlike Buddhism this is about the self (Buddhism concludes the transience and impermanence of the self). The soul of humankind is to be developed and raised through Karma so that it can be joined into God (whereas for Buddhism there is no God - thus impermanence). The soul may go through many a body, but it is indestructible and through meditation, action and devotion is to be raised.
Despite many influences, Hindusim remains a combination of caste and tolerance, of hierarchy and diversity of religious practice and beliefs.
After watching a BBC programme by Girish Kamad, 10/08/2002