This section is from the book "Some Contributions Of South India To Indian Culture", by S. Krishnaswami Aiyangar. Also available from Amazon: Some Contributions of South India to Indian Culture.
A theistic system of Bhakti consists in the worship of a personal God who is the Creator and Lord of the Universe. Devotion to him by unremitting service is the best way to the attainment of salvation or release from the ever-recurring cycle of births and deaths, and as such become recognisable as a system in the age of religious ferment of the Upanishads. The natural development of this religious ferment led to the rising of Buddhism and Jainism in the east. A more legitimate and orthodox system also grew simultaneously in the home of orthodoxy in the middle country of Hinduism. This system is represented both in the Narayaniya section of the Mahabharata as well as in the Bhagavat Gita. Both of these Sir R. G. Bhandarkar traces to a period anterior to the rise even of Buddhism and Jainism, but not in an organised form. He would regard the Gita as a system which came into existence as a protest against the atheistic systems which resulted from the intellectual ferment of the age of the Upanishads. The following is Sir R. G. Bhandarkar's summary of the position: "The state of things which must have led to the evolution of the religion of the Gita seems to me to be this. About the time when the systems of religion we have been considering arose, there was a tendency amongst the people which often worked itself out, as is evident from the Pali birth-stories, to give up worldly life and betake themselves to residence in forests or mountains. Even Buddhism, Jainism and other like systems considered an ascetic life to be a sine qua non of religious elevation. There is reason to believe that Sramanas existed before the rise of Buddhism. The religious systems that had sprung up were mostly atheistic. The Indian mind had become prone to indulge in mere moral discourses and thoughts on moral exaltation, unassociated with a theistic faith as appears clear from Buddhism and other systems, and also very dry moral dissertations of which the Mahabharata is full. Such a system as that of the Bhagavat Gita was therefore necessary to counteract these tendencies. Theistic ideas were so scattered in the Upanishads, that it was necessary for practical purposes to work them up in a system of redemption capable of being grasped easily. These appeared to be the conditions under which the Gita came into existence.
I am not inclined to dissolve Vasudeva and Arjuna into solar myths; Vasudeva could not have been living when the Bhagavat Gita was composed as a discourse delivered by him, any more than Buddha was living when his discourses were reduced to the form of books. It is worthy of remark that both of them are called Bhagavats when speaking. Vasudeva must already have been deified before the Bhagavat Gita was written." The School of Bhakti therefore can go back to Vedic beginnings reaching back to the Upanishads certainly and may be traced even anterior to this particular stage of development. As a system it may be regarded as pre-Buddhistic judged by the Gita alone. Bhakti consists as was already pointed out in love of God and complete devotion to Him. Such a notion is traceable in some of the Upanishads themselves. As a system the school of Bhakti regards Vasudeva as the supreme soul, the internal soul of all souls. He is regarded as the supreme creator. All living beings are represented by Sankarshana, who is a form of Vasudeva. From Sankarshana sprang Pradyumna, the mind and from Pra-dyumna, Anirudha, self-consciousness. Prom him sprang Brahma. The first four are regarded as the four Vyuhas of the supreme. A similar hier archy of gods is found in connection with the school of the Tantra only instead of Vasudeva, Mahesvara has to be substituted and perhaps even behind this the great mother or Parasakti. On a detailed consideration of references in literature Sir R. G. Bhandarkar arrives at the conclusion "Still it is doubtful, and it may be taken for granted that the two Vyuhas Vasudeva and Sankarshana only were known up to the time of the earliest inscription which is to be referred to about the beginning of the first century before the Christian era, so that the system of four Vyuhas was not fully developed up to that time." Prom this he draws the further inference that as the Bhagavat Gita has no specific reference to the four Vyuhas it must have been composed at a time anterior to this period and to a period up to which we could trace references to the Vyuhas in literature reaching back to the 4th century B. C. The worship of Vasudeva and Baladeva, among the very large number of deities including even animals and trees, is referred to in a passage in the Buddhistic Niddesa1 referable to the 4th century B. C. There is a reference to a shrine to Sankarshana in the Arthasastra. Patanjali refers to Vasudeva as God in his comment on Panini IV, 3, 98. In an inscription at Ghasundi in Rajputana there is a reference to the temple for Sankarshana and Vasudeva. This inscription is dated about 200 B. C. on Palgeographical grounds alone.1 The Besnagar inscription refers to the erection of a Garudadhvaja "in honour of Vasudeva the God of Gods." That was constructed by one who bore a Greek name Helio-dora who describes himself as the son of Diya and as a Bhagavata. He further states that he was a native of Takshasila and was an ambassador of the Yavana Antalikita to Bhagabhadra, probably ruler of eastern Malva. This inscription is referable to the second century before Christ. There is a reference to Sankarshana and Vasudeva in the Nanaghat2 inscription No. 1, dated the first century of the Christian era on palaeographical grounds. The particular way in which the name Vasudeva occurs in the Sutra of Panini and the explanation that Patanjali offers support, the presumption that Vasudeva was regarded as a divine person even in the days of Panini. Clearly historical references therefore take back the worship of Sankarshana-Vasudeva to the 4th century B.C. Sir R. G. Bhandarkar however doubts whether the four Vyuhas were known so early and concludes "It may be taken for granted that the two Vyuhas, Vasudeva and Sankarshana only were known up to the time of the latest inscription which is to be referred to about the beginning of the first century before the Christian era, so that the system of four Vyuhas was not fully developed up to that time."1 Apart from these however the learned doctor would regard the essential teaching of the Bhakti school traceable in the Upanishads themselves. He gives reference to two passages from the Upanishads, which contain according to him, "a verse to the effect that this supreme soul is not to be attained by lectures (from a teacher), nor by intelligence nor by much learning; He is to be attained by him whom the supreme soul favours; to him he discloses his form. Again we have the doctrine that the supremely wise Being, the life of all, leads a man to do good deeds, whom he desires to elevate (K. U. II, 8); and another that God dwelling in the heart of all beings controls them which latter forms the subject of a celebrated passage in B. U. 1ll, 7. From this it is clear that the doctrine that the individual soul is dependent on the Supreme and that the latter alone works out the salvation was acknowledged in Upanishadic times." We shall show later on that this is exactly the doctrine of the Southern school of Bhakti.
1 Sir R. G. Bhandarkar's Vaiahnavism, etc., p. 3.
1 Ludder's List of Brahmi Inscriptions, No. 6.
2 Ladder's List of Brahmi Inscriptions, No 1112.
1 On the whole of this see Mr. R. P. Chanda: Arch. Memoirs, No. 5, 1920.