The Buddha was very fortunate actually to have come on the scene a century or so after the golden age of the Upanishads, as there was a background of religious feeling and understanding which enabled many people to appreciate the depth of his teaching.
Buddhist Asia
Esoteric Buddhism and the Tantras in East Asia
Some scholars use esoteric Buddhism and tantra as virtually interchangeable, generic terms covering distinctive developments all across Buddhist Asia from the third or fourth centuries C.E. onward…