The central change of consciousness that underlies all of the Gita’s teachings is one that moves us from the normal human state of seeing everything in a state of fragmentation, division and separation, to one that sees all existence as one unified whole or unified field. The implications of this transformation lead to a large number of consequences both in terms of how we see the world and how we relate to the world.
The Gita did not originate this perception. The Upanishads have the consciousness of Oneness as a constant theme. For instance, this remarkable verse from the Taittiriya Upanishad: “The Spirit who is here in man and the Spirit who is there in the Sun, lo, it is One Spirit and there is no other.” (Bhriguvalli Chapter 10) as well as “…for when the Spirit that is within us findeth his refuge and firm foundation in the Invisible, Bodiless, Undefinable and Unhoused Eternal, then he hat passed beyond the reach of Fear. But when the Spirit that is within us maketh for himself even a little difference in the Eternal, then he hath fear, yea, the Eternal himself becometh a terror to such a knower who thinketh not.” (Brahmanandavalli Chapter 7)
Similarly the Isha Upanishad: “He in whom it is the Self-Being that has become all existences that are Becomings, for he has the perfect knowledge, how shall he be deluded, whence shall he have grief, who sees everywhere oneness?” (Isha Upanishad, v. 7)
Sri Aurobindo summarizes the Gita’s approach: “Oneness with God, oneness with all beings, the realisation of the eternal divine unity everywhere and the drawing onwards of men towards that oneness are the law of life which arises from the teachings of the Gita.” “This freedom and this oneness are the secret goal of our human nature and the ultimate will in the existence of the race.” “To be made one self with God above and God in man and God in the world is the sense of liberation and the secret of perfection.”
Sri Aurobindo, Essays on the Gita, First Series, Chapter 20, Equality and Knowledge, pg. 201
Sri Aurobindo, The Upanishads , Taittiriya Upanishad, pages 271, 281 and Isha Upanishad, page 21