The Srimad-Bhagavad-Gita, the Essence of the Upanishads, the Science of Brahman, the Scripture of Yoga, the Dialogue between Sri Krishna and Arjuna.
Even in very early times there had existed, side by side with the hieratic Vedic religion, several popular, tribal religions. The gods and goddesses of these tribal people differed from the divinities of the official Vedic pantheon, and the religious practices associated with them also differed fundamentally from the religious practices of the Vedic Aryans. Nevertheless, these indigenous religious eventually found a place under the broad mantle of the Vedic religion. While Brahmanism remained in the ascendancy, their sphere of influence was restricted to the tribes among which they had originated. But the gradual decline of Brahmanism, in the face of competition from Buddhism and Jainism, afforded these popular religions an opportunity to assert themselves; and indeed, the Brahmanists themselves seem to have encouraged this development to some extent as a means of meeting the challenge of the more heterodox movements. At the same time, among the indigenous religions, with their variety of gods and relgious practices, a common allegiance to the authority of the Veda provided a thin, but nonetheless significant, thread of unity. This is the genesis of Hinduism, which brought together under its banner large masses of people, and, at the same time, kept the Vedic tradition alive.
One significant constitutent of this all-embracing Hinduism was Krishnaism, which seems to have originated and spread among the tribes of Western and Central India, like the Vrishnis, the Satvatas, the Abhiras, and the Yadavas. Its principal teacher was Krishna, who was associated with the above-mentioned tribes as either their temporal or their spiritual leader, and was in course of time, transformed into a tribal god. That this tribal god and the religious movement inspired by him were originally not countenanced by the Vedic religion is suggested by the episode at the Govardhana mountain (Harivamsa, 72-73), which describes the antagonism to and subsequent subjugation by Krishna of the chief Vedic god Indra. This is clearly symbolic of the growing predominance of the popular religion over the hieratic Brahmanic religion. But the religion of Krishna typifies the paradoxical characteristic of Hinduism mentioned above, namely, that it was a fundamental departure from Brahmanism which nonetheless remained within the bounds of loyalty to the Veda. ...
The religion of Krishna differed from from the Upanishads, as well as from Buddhism and Jainism, first and foremost in its teaching about the goal of human life. The Upanishads generally put forth the view that, since this phenomenal world and human existence are in some sense unreal, man should renounce this worldly life and aim at realizing the essential identity of his soul with the Universal Self, which is the one and only absolute reality. The Upanishadic attitude toward life and society is fundamentally individualistic. The Gita, on the other hand, teaches that man has a duty to promote lokasangraha or the stability, solidarity, and progress of society. Society can function properly only on the principle of ethical interdpendence of its various constituents. As an essential constituent of society, therefore, man must have an active awareness of his social obligations. The sva-dharma (lit. one's own dharma, set of duties) or the specific social obligations of different types of men are, according to the Gita, best embodied in the doctrine of the four classes. The Gita, however, emphasizes the metaphysical significance of that scheme, according to which all classes are equal and essential, while it insists mainly on man's active recognition of sva-dharma or his own specific social obligations.
The second fundamental point on which the Gita differs from Upanishadic thought follows logically from the first. The Upanishadic ideal of spiritual emanicpation through knowledge involves the acceptance of the unreal character of the phenomenal world. Through his actions, consciously or unconsciously, man becomes involved in the tentacles of this fictious world and is thus removed farther and farther from his goal. A complete abnegation of action, therefore, came to be regarded almost as a sine qua non of a true seeker's spiritual quest. The ideal of social integrity (lokasangraha) through sva-dharma enjoined by the Gita, on the other hand, implies an active way of life. The Gita, indeed, most often speaks in terms of yoga (application to work or self-discipline) rather than of moksha (release or liberation). The teacher of the Gita has discussed, at great length, the why and the how of the yoga of action (karma-yoga). The activism inculcated by the Gita is, however, not of the common variety. It is tinged ‹ perhaps under the influence of Upanishadic and Buddhist thought ‹ with an element of renunciation. It argues that action, as such, is not detrimental to one's attainment of his spiritual goal. It is only one's attachment to the fruits of action that keeps one eternally involved in the cycle of birth and death. The Gita, therefore, teaches the art of "acting and yet not acting," i.e., acting without becoming personally involved in the action.
Whereas Vedic ritual practices were exclusive in character, Krishna sponsors a way of spiritual life in which all can participate. It is the yoga of devotion (bhakti yoga). In contrast to ritual sacrifice the Gita offers a concept of sacrifice embracing all actions done in fulfillment of one's sva-dharma and without attachment to their fruits. This way of devotion presupposes the recognition of a personal god ‹ in the present context, of course, Krishna himself ‹ who is regarded as being responsible for the creation, preservation, and destruction of the universe. The devotee serves that God like a loyal servant, always craving some kind of personal communion with Him. The criterion of true worship, according to the doctrine of devotion, is not the richness or profuseness of the materials used for worship nor the number and variety of religious observances involved in it. It is rather the earnestness, the faith, and the sense of complete surrended to the Divine on the part of the devotee (bhakta). Such a devotee ‹ whatever his age, sex, learning, and social status ‹ compels God to become his friend, guide, and philosopher. The way of devotion is thus more simple, more direct, and more effective than any other religious practice. To this teaching of devotion, however, Krishna makes one significant addition. He insists that a true practitioner of the yoga of action (karmayogin) also become a true devotee, for, by following his own duty (sva-dharma), the karmayogin is doing the will of God and participating in the divine project.
Krishnaism cannot boast of any independent philsophical system of its own. The great virtue of the Gita is that, instead of dilating upon the points of difference among the varius systems of thought and practice, it emphasizes the points of agreement among them and thereby brings about a philosophical and religious synthesis. We have already suggested that the Gita underwent a kind of Brahmanic reorientation. One of the more significant results of this reorientation, as far as th epersonality of Krishna is concerned, was that this tribal god, who was essentially non-Vedic in origin and whose character had already become syncretic, came to be regarded as an avatar (incarnation) of the Vedic god Vishnu, and as identical with the Upanishadic Brahman.