But of course, we cannot say that Buddhism was not influenced by Vedic Brahmanism and later Hinduism at all. That would be too naïve. However, in the give and take which is inevitable in any culture within a space of time (and Buddhism covered 75% of India and 75% Asia for sixteen or so hundred years), it was Hinduism which took mostly from Buddhism and not the other way around.
Another interrelated myth is that it was Sankaracharya who defeated the Buddhists all over India and that is how Buddhism vanished from India or as the former President of India Dr. Radhakrishnan Sarvapulli put it, Hinduism embraced Buddhism and in the process killed it. Again these are myths running wild amongst Hindus of the Indian sub-continent; but they do not have any historical validity. This notion is given further credence to Nepalese, including Buddhist Newars by the Newari legend that Sankaracharya came to Nepal and defeated all the Buddhists, converted the kings and beheaded the Bhikchhus. First of all the Adi Sankaracharya was around the 7th century and great Mahavihars like Nalanda and Bikramashila were still running strong till the 12th /13th century when the Muslims over ran India and destroyed them.
Secondly there were still Mahasiddhas like Naropa, Tilopa and many others till the Muslim invasion. So, Buddhism was still running strong five century after Adi Sankaracharya. And furthermore, the stories of Sankaracharya as written by Ananda Giri and Madhava etc. do not contain any element which mentions that he debated with the Buddhists all over India and defeated them. In fact those stories show Sankaracharya debating mostly with other non-advaita Hindus and rarely with the Buddhist. So, the misconception that Sankaracharya went up and down India defeating all the Buddhists and this is how Buddhism vanished from India seems to be baseless and fabricated by uneducated Non-Buddhists.
Thirdly, the Sankaracharya that came to Nepal seems to be of the 11th-12th century or later and not the Adi Sankaracharya. He seems to have entered Nepal when Buddhism was beginning to decline in Nepal as a result of its having declined in India due to the Islamic invasion which literally destroyed Buddhism in India. So he did not find any match for his debates and was able to convert many people in Kathmandu. He may possibly be the same Harinanda who was defeated by the Great Tibetan Guru Sakya Pandit. However this is not conclusive. But the stories do say he died in Tibet. However he did not die before he created havoc amongst the Buddhists of Kathmandu Valley, who still do not seem to have recovered from the shock. Big learning houses like Nalanda, Bikramashila etc were raised to the ground and the monks beheaded and the books in the libraries burnt to cinders by the Islamic invaders like Bakhtiar, Khilji etc.
It is said in the diary of Khilji's general that, the books of the library of Nalanda took six months for the cinders to settle down and nine months for the smoke to settle down. So much destruction took place all over the Indian subcontinent. It said one of the reasons why the Buddhist monasteries were specially picked out by the Islamic invaders is that they mistook the monks in uniform monk dress as uniformed army men and the books in the library as books on warfare et al. This happened in the 12th/13th century, almost 5 centuries after Sankaracharya. Till then Buddhism was still flourishing strong in the Indian subcontinent.
Yes the Adi Sankaracharya refuted the Buddhist tenets in his commentaries of the Upanishads and Brahma Sutra; but the Buddhists have also equally refuted the concepts of Sankara. Debate and refutation was both ways till the Islamic Invasion. It was only after Buddhism was literally raised to the ground by the Islamic Invaders that present day Hinduism, which is a metamorphosed form of Vedic Hinduism, began to raise its head. Till then 75% of Indian subcontinent and 75% Asia was Buddhist. From the time of the Buddha and specially from the 1st/2nd century till the 11th/12th century, when the Vajrayana form of Buddhism was in sway, Buddhist art, philosophy and logic developed to its fullest potential. It can certainly be said that, that was the golden period of Indian culture as a whole and Indian Buddhism specifically.
This was also the period when, as a result of interaction with Buddhism, Hinduism also developed to its cream. It should be remembered that Sankaracharya who is considered as the cream of Hinduism by an overwhelming majority of the Hindus, was a product of the 6/7th century and many ancient Hindus like Bhaskaracharya etc even called him pracchanna Bauddha (crypto Buddhist). Why did these Hindu pillars call Sankaracharya a crypto-Buddhist? This is not because he, his philosophy or tenets were like the Buddhists'. No, far from it, he has attempted to refute the Buddhist tenets. It is because he has used the Buddhist logical modus operandii to refute all his opponents which included the Hindus, Buddhists and Jains. This clearly shows how even Sankara was influenced by Buddhism. The great Buddhist Nyaiyayik (logician) Dharmakirti literally changed the logical system of the Indian subcontinent with his Buddhist logical tenets.
Another big confusion is that the Buddhist Tantra was a result of the influence of Hindu Tantra on Buddhism. But the famous Indian Iconographist Benoytosh Bhattacharya has amply proven that it is the other way around. Hindu Tantra developed after Buddhist Tantra (Vajrayana) reached its acme in the Indian subcontinent. One of the oldest Hindu Tantric literature the Pichu Tantra also called the Rudrayamala and the Brahmayamala very clearly states that Vasistha went to Mahachina (Tibet) to study the tantric methods with Shiva-rupi Buddha.
Now till the 12th century, Tibetans came down to the hot plains of India to study the tenets of Vajrayana in the great learning houses like Nalanda/Bikramashila etc. Now this means this oldest Hindu Tantra was written after the 12th century and not before that. It was written after Vajrayana vanished from India after the Islamic Invasion. Although Hindu Tantra developed as a result of the influence of Vajrayana on the entire subcontinent, the two are only apparently similar. A deeper probe into both of them exposes a tremendous difference not only of the paradigms on which each is based but also on the principles on which each is based, the path followed by each and the final goal of each.
The entire Hindu Tantric systems are themselves diverse; some based on Shakti, others on Shiva and some on Visnu. The objective of most of them is to unite with the deity and finally attain Brahma, Parasamvit or Sambhava states. Excepting the dualistic tantras, they are all varieties of advaita Vedanta where other names substitute the Brahma of the Vedanta. Most of them are geared towards the realization of the Eternal unchanging self called the Atma in the entire Hinduistic system. Now the whole of Buddhist Tantra is geared to the realization of emptiness (sunyata) which is a subtle form of Anatma.
Hinduistic Tantra is based on the experience of an eternally existing, unchanging entity called the true Self or true Atman, whereas the entire Buddhist Tantra is based on the experience that from the very beginning there is no eternally existing, unchanging Self. Both experience is a non-dual experience. In the Hindu system one merges non-dually with the eternal, unchanging Self and that is the non-dual experience. In Buddhist Tantra one sees through that there is no eternal, unchanging Self as opposed to the changing world. So there is no two, i.e. advaya. Many scholars have been confused by similar words like advaita/advaya and many others used in both the systems and believe that they are two versions of the same thing. Nothing could be further away from the truth.
There are also many differences in the path; but that would require detailed technical nitty gritties which is not the purpose of this article. So we shall stop here about these points. All forms of Mahayana Buddhism within which Vajrayana lies, uses Sanskrit as its lingua franca. Since Hinduism and Hindu Tantra also uses Sanskrit, and because Buddhism and Hinduism developed first and foremost within the cultural milieu of the Indian subcontinent, it is not surprising that similar words are used in both system. For example, words like mantra, dhyana, Samadhi are common to both but do not necessarily mean exactly the same thing and one must not be fooled by the use of such common words to conclude that Buddhism and Hinduism are the same.
One famous Nepalese Brahmin scholar saw that the word Bhairava is used in the mantra of Bignantak and used that as a proof that the Buddhist worship Bhairava and thus they are the same. In the Buddhist context the word only means wrathful and not any particular deity as is the case in Hinduism. The two tantric systems of the Indian subcontinent are as different from each other as Theravada is from Vaisnavism. Only the name Tantra is the same but even the exact definition of tantra in each of the system is drastically different. So these are some of the myths about Buddhism rampant amongst non-buddhists of Nepal which needed to be exploded.
These rampant confusions exist amongst the non – buddhists of the Indian subcontinent because, it has been over nine centuries since Buddhism was erased from the memory of the Indian subcontinent. It is common place for absurd rumors to spread like wild fire in the absence of authentic information.
The people of the Indian subcontinent came to believe that Buddhism had died out completely and did not exist at all; so each was free to interpret it according to one's own predilictions. But in reality Buddhism continued to survive in full fledge in other lands where it was taken by the inhabitants of the Indian subcontinent themselves. Buddhism is still alive and dynamic in Central Asia, Mongolia, Tibet, China, Korea, Japan, Bhutan, Vietnam, Laos, Cambodia, Thailand, Burma, Sri Lanka, the Cis – Himalayan regions of Nepal and India, and in the Kathmandu Valley.
But remarkably enough blinded by their own cultural preconceptions, biases and prejudices even the non – buddhists of the Kathmandu Valley who could not but rub shoulders with it constantly, were completely oblivious about its reality and continued to subscribe to the rumors made up by their Indian Gurus. This is indeed one of the world's best epitome of how blind spots control the human mind, that in Nepal where Buddhism never died, the non – buddhist populace virtually know nil about authentic Buddhism.
The Indians would of course have their own theories of how Buddhism disappeared from India. However, the truth of the matter is due to the Muslim invasion that destroyed the great monasteries in India and the already pervalent degeneration of some of the bigger monastic institutions like Nalanda and Vikramashila.
Actually, it is due to the degeneration of the Sangha that spurred the further development of tantrism which saw its adherents as the divinely mad mahasiddhas like Naropa, Tilopa and so forth. Anyway, without the monasteries and great instutitions educating the next generation of teachers, scholars and saints, there's no way the general populace would be able to differentiate Buddhism and Hinduism as most of them are illiterate anyway.