All we can say with certainty is, that when Fā-hien visited Mathurā on the Jumnā 400 years after Christ, their cult certainly existed there at that time.
We shall not be far wrong if we assert that it was adopted in about the third century of our era.
As already indicated (see [p. 175]), the idea of the first Buddhist triad—the Buddha, the Law, and the Monastic Order—accepted by the adherents of both Vehicles—was probably derived from the earliest Brāhmanical triad. (See also Brāhmanism and Hindūism, pp. 9, 44. 74.)
In the same way the second Buddhist triad introduced by the advanced teachers of the ‘Great Vehicle,’ viz. Mañju-ṡrī, Avalokiteṡvara (= Padma-pāṇi), and Vajra-pāṇi, corresponded to the later Hindū triad (tri-mūrti) of deities, Brahmā, Vishṇu, and Ṡiva.
I have explained in ‘Brāhmanism and Hindūism’ (pp. 54, 73) how the gods Vishṇu and Ṡiva gradually usurped the position of the god Brahmā, whom they dispossessed of his co-equality with themselves, and how the whole mythology of the Hindūs, which was originally complicated by a large admixture of pre-Āryan and Vedic elements, ultimately became more simplified by arranging itself under the two heads of Vaishṇavism and Ṡaivism, all other mythological personages being regarded as forms of either Vishṇu or Ṡiva.
In contradistinction to this, we find that each member of the two Buddhist triads holds its own, and we are led on to a system which bewilders us by ever increasing complications—a system which preserves the individuality of its own triads and deified saints, and yet recognizes almost all the gods, demigods, demons, and supernatural beings of Hindūism.
I propose now to offer some account of the development of the Buddhist Pantheon, beginning with the mythical conceptions peculiar to Buddhism, and passing on to those held in common with Hindūism.
And first as to the second Buddhist triad above-named, it may be noted, as a proof of the very gradual growth of Buddhistic mythology, that in the earlier developments of Buddhism the three Bodhi-sattvas constituting that triad have very restricted functions.
When I visited the Buddhist caves at Ellora, I noticed that in the ancient sculptures there, Padma-pāṇi and Vajra-pāṇi (but not Mañju-ṡrī) are represented as attendants of the human Buddha.
Of course it is easy to understand that the duty of guarding the Buddha ultimately expanded into that of watching over and protecting the whole Buddhist world, though it is difficult to determine which of the three mythical Bodhi-sattvas became first celebrated for the effective discharge of this duty, or to which of the three chronological precedence ought to be assigned.