Bhagavad Gita propagates the idea not only of oneness of mankind, but also of the unity of all beings. It emphasises oneness of the Atman with all life and of all existences with the Divine.
While describing the Supreme Brahman, it has been stated in shloka 13.17 that though appearing as divided among various beings, the Supreme is indivisible. He is the one who is supporting all beings, destroying them ; and creating them afresh.
Thus it is evident that all beings arise from the Supreme, are supported by Him and at the time of dissolution are withdrawn into Him. All beings exist in the Supreme. No one stands apart from Him.
Though appearing as divided into multiplicity of beings, the Supreme remains an integral and undivided reality. He is the essence of all creatures. Unity of all beings is, thus, the truth and the apparent multiplicity is an expression of that eternal truth.
Oneness of the whole stream of life has been re-iterated in shloka 13.31 wherein it is stated that when one sees different beings as rooted in the One (Supersoul) ; and everyone spreading out from that source alone, he then attains to the Absolute (Brahman).
In other words, when one understands that the Supersoul, Pramatman, is ever present as individual soul, jivatman, in each and every being ; he then realises that the diverse variety of living beings are situated in the Eternal One and are born from that source alone. After gaining such spiritual vision, one attains to the Brahman i.e. becomes identified with the Absolute. He then realises the true nature of the ultimate reality which is nothing else but eternal bliss.
Origin and dissolution of all beings has been analysed in shlokas 7.4 to 7.6. As described in these shlokas, the Supreme Lord, Ishvara has two types of natures namely the material nature (apara prakriti – the lower nature) and the conscious nature (para prakriti – the higher nature). Everyone takes birth in these two natures. Ishvara is the origin as well as dissolution of whole of the world. Corresponding to the above mentioned prakritis of Ishvara, each individual has two aspects of life, namely the mind-body complex and the soul. Both of these are derived from Him. Source and destination of everyone, thus, remains the same.
Oneness of the whole stream of life has also been enunciated in shloka 5.18 wherein it is stated that a wise person sees all – a learned and humble brahmin, a cow, an elephant, a dog or an outcast – with an equal eye.
In other words, the one endowed with wisdom realises that all beings are different manifestations of God. He understands that the eternal and unchanging reality behind everybody is the same. Hence, all differences of high and low or even of man and animal vanish. One regards everyone equally.
The theme of oneness of the numerous forms of life has also been stressed in shloka 6.29 wherein it is stated that he whose self is disciplined in yoga (yog-yukt-atman), sees the self abiding in all beings and all beings existing in the self ; everywhere he sees the same. To put it simply, when enlightened by yoga, one understands that the same Atman dwells in him and in everyone else. He also realises that everyone is situated in the same Atman. He establishes his oneness with all life.
Not only oneness of the whole flow of life, Krishna has also revealed the unity of all existences. In shloka 6.30, it has been stated that he who sees the Supreme Lord everywhere and sees everything in Him, he never loses sight of Him nor is he ever out of His sight. This shloka emphasises on the profound unity of everything in the all-pervading Ishvara. He remains established in the hearts of those who realise their oneness with Him.
Knowledge about the unity of all beings has been declared to be the true knowledge. It has been stated in shloka 18.20 that the knowledge by which all beings, though appearing divided in different forms, are seen as indivisible and imperishable reality, such knowledge is in the mode of goodness (sattvic gyana).
When freed from all-enveloping darkness of ignorance, one gets true light of knowledge. He understands that every jivatman, the individual soul, is an integral part of Pramatman, the Supersoul, in the same manner as every wave is an inalienable part of the sea. The sameness underlying the multiplicity of creation dawns upon him. He percieves the undivided in the divided and realises the supreme unity of all existences. He is then able to see the Supreme in all and all in the Supreme.
But to recognise the inherent oneness of all existences is not to deny the fact of empirical world, the world of our observance and experience. The physical world of myriad distinctions cannot be and need not be negated, for it is not an illusion. The multiplicity of creation is, in fact, an expression of the material nature (apara prakriti) of the Supreme. But at the same time, one must never forget that the ultimate reality of everyone is the same. The Supreme is present equally in all beings.
When one realises that all of us are sustained by the benevolence of God, he becomes more humane. Once such an attitude is developed, one is a sage, even though fully engaged in all kinds of social and economic activities.