Bhagavad-Gita, Song of the Lord

One commander in the Mahabharata battle, Arjuna, the third Pandava brother, persuades his friend Krishna to be the driver of his war-chariot. Now Krishna is no ordinary man; he is God, come to earth in human form; but, as often happens in Hindu stories, he stoops to serve his devotee. Arjuna, face to face with his opposing kinsmen on the battlefield, suddenly realizes that, wicked though they are, it is he who must kill them. His mouth dries up, his bow falls from his hand. "How can I kill these," he exclaims to Shri Krishna, "my cousins and uncles, my teachers and friends? Better for me if I die myself! "Krishna reminds him that his cause is just; that he has been born and trained a warrior, whose duty is to fight. No action is absolutely right or wrong; our motive and attachment make us judge it so. He says, "Shake off this fever of ignorance. Stop hoping for worldly rewards. Fix your mind on the Self. Be free from the sense of ego. Dedicate all your actions to me. Then go forward and fight." There, between the two armies, Shri Krishna in eighteen chapters spells out his argument, and in so doing describes in memorable passages all the yogas and how to practise them. It is no wonder the Gita is by far the favourite of all Hindu scriptures.
Most important of all, Krishna gives Arjuna an experience to clinch his arguments: he shows him that in God's vision this battle is already fought and won; Arjuna is merely to be an instrument. And he reveals to him, that he, Krishna, is the Lord Himself.
"When goodness grows weak and evil increases, I make myself a body. In every age I come back to deliver the holy, to destroy the sin of the sinners, to establish righteousness."