They were Buddhists, and it is probable that the Mahayana or northern form of Buddhism was due to an amalgamation of Gotama's doctrines with the ideas (largely Greek and Persian) which they brought with them.
And as the Mahavastu was a standard work of a particular sect, or rather school, called the Maha-sanghikas, it has thus preserved for us the theory of the Buddha as held outside the followers of the canon, by those whose views developed, in after centuries, into the Mahayana or modern form of Buddhism in India.
The latter, in Sanskrit, is the earliest exposition we have of the later Mahayana doctrine.
We have now also the text of the Prajna Paramita, a later treatise on the Mahayana system, which in time entirely replaced in India the original doctrines.
Thus in Buddhism the presuppositions which Buddha uncritically took over work out their logical results in the Mahayana, so that great sects calling themselves " Buddhist " affirm what the Master denied and deny what he taught.
The Mahayana systems are the union of Buddha's teaching with the forms of the Brahman philosophy.
In the Mahayana gnosticism was triumphant, and the historic values of Gautama's teaching and personality are lost.
In the opening words of each Mahayana treatise a list is given of such Bodhisats, who were beginning, together with the historical Bodhisats, to occupy a position in the Buddhist church of those times similar to that occupied by the saints in the corresponding period of the history of Christianity in the Church of Rome.
But the great bulk of the collection consists of Mahayana books, belonging to all the previously existing varieties of that widely extended Buddhist sect; and, as the Sanskrit originals of many of these writings are now lost, the Tibetan translations will be of great value, not only for the history of Lamaism, but also for the history of the later forms of Indian Buddhism.
The long period of depression seems not to have been without a beneficial influence on the persecuted Buddhist church, for these teachers are reported to have placed the Tantra system more in the background, and to have adhered more strongly to the purer forms of the Mahayana development of the ancient faith.
In doctrine the great Tibetan teacher, who had no access to the Pali Pitakas, adhered in the main to the purer forms of the Mahayana school; in questions of church government he took little part, and did not dispute the titular supremacy of the Sakya Lamas.
The Mahayana illustrates in part what would have followed the .triumph of gnosticism in Christianity, for not only would the historic value of the life and teaching of Jesus have been lost, but with it the significance of humanity.