Gautama Buddha was a Hindu reformer in a time of corrupt priests. Like Jesus, he had no intention of creating a new religion. He was bringing new understandings to the teachings of the Vedas and Upanishads. Sadguru Sant Keshavadas and Sadguru Rama Mata taught us that Gautama Buddha was the 9th Avatar of Vishnu.
The Asian area between India and China had been Hindu for several millennia. Naturally, they added the Buddhist teachings to its predecessors. This should not be hard to understand for Westerners, because Christians continue to believe in and teach the Torah, the Old Testament.
The idea that Gautama Buddha was an atheist is nonsense. He taught a version of Hinduism that freed people from the corrupt priests. His major change from the teachings of the 8th Avatar, Krishna, was that he taught spiritual independence. Krishna taught people to depend on him, to merge their consciousness in him. Gautama Buddha taught that our sufferings are our own creations and that we can end our sufferings by ending our cravings — those attitudes and behaviors that create karma.