Buddhism is not Hindu in origin
It is frequently claimed that Hinduism is the oldest religion in the world because it is loosely based on the Vedas. It is also claimed that Siddhārtha Gautama founded Buddhism in response to Hinduism (or, possibly even as a reformation of Brahmanical/Vedic Hinduism). It is assumed that the Buddha was a Hindu in the same way that Jesus was a Jew. This, however, is not accurate at all. In fact, Theravada Buddhism is older than modern Hinduism and Buddhism is the polar opposite of Hinduism in many ways. Buddhism can’t accurately be viewed as a reformed Vedic religion.
Hinduism, as it exists today, wasn’t really around in the time of Siddhārtha Gautama (the Buddha). While Hinduism is loosely based on the ancient Vedic texts (Vedas, Brahmanas, and Upanishads), most of modern Hinduism is a much more recent development. The largest sects in modern Hinduism are Vaishnavism, Shaivism, and Shaktism but Vishnu, Shiva, and Shakti don’t actually play a significant role in the Vedas. In the Vedas, the major deities are Agni, Indra, and Soma, whereas Vishnu only has a minor role while Shiva and Shakti are never even mentioned. Shakti, Durga, and Kali are either not mentioned directly in the Vedas or play only minor roles in comparison to the major goddesses Ushas, Aditi, Prithvi, and Saraswati. The main gods of the Vedas (Agni, Indra, and Soma) are not even worshipped in modern Hinduism. The main goddesses of the Vedas, with the exception of Saraswati, aren’t worshipped in modern “Hinduism” either. Furthermore, the rituals recorded in the Vedas entail the consumption of soma, an entheogenic drink, but this ritual is not performed by modern brahmins in Hinduism. And the other Vedic rituals entail animal sacrifices, a practice prohibited by Vaishnavism, the largest sect of modern Hinduism. So, the Vedic religion that existed in the time of Gautama Buddha is not the same as modern Hinduism by any means. Modern Hindus don’t worship the same gods, perform the same rituals, or even have the same beliefs as the ancient Vedic Brahmins did. Modern Hinduism was really just beginning to develop into a distinct religion during the Buddha’s lifetime.
And the Vedas actually are not the oldest scriptures in the world unless you accept the Brahmanical claim that they are eternal. The Epic of Gilgamesh is older. Furthermore, the Vedic belief system emerged from an older Indo-Aryan religion, the same religion from which Zoroastrianism and various ancient Middle Eastern religions evolved.
In addition to all of that, a lot of what we think of as Hindu philosophy and theology is a relatively recent development as well. A lot of ideas that we associate with Hinduism come from the 8th Century philosopher Adi Shankara, who was actually heavily influenced by the ideas of Buddhist philosophers like Nagarjuna. In fact, Shankara was so influenced by Buddhist philosophy that Ramanuja accused him of being a crypto-Buddhist. The main philosophical schools in modern Hinduism are Advaita Vedanta (founded by Adi Shankara in the 8th Century CE), Dvaita Vedanta (founded by Madhva in the 13th Century CE), Vishishtadvaita Vedanta (founded by Ramanuja in the 11th Century CE), and Achintya-Bheda-Abheda (founded by Chaitanya Mahaprabhu in the 15th Century, though it has a precursor in the 8th Century philosopher Bhāskara). These Hindu schools of thought often define themselves in contrast to Buddhism and couldn’t have really emerged in history until after certain developments within Buddhist philosophy had occurred.
So, Hinduism as we know it didn’t really exist at the time that Buddha was teaching. The cults of Vishnu, Shiva, and Shakti were just beginning to form at the time and the philosophy/theology of modern Hinduism wouldn’t come into being for well over a millennia. The relationship of Hinduism to the ancient Vedic religion that was around at the time of Gautama Buddha is akin to the relationship of Roman Catholicism to the religion of Abraham — it’s anachronistic and inaccurate to say that Abraham’s religion was Roman Catholicism (Abraham certainly never heard of the Trinity or of the Pope!), so too it would be inaccurate to say that the religion of the Vedic Brahmins of Buddha’s day was Hinduism (they were worshipping entirely different deities and didn’t have the same philosophical-theological beliefs either!).
Furthermore, the religious context of the society in which Gautama Buddha lived was mostly non-Vedic. Siddhartha Gautama was a member of the Shakya (Scythian?) clan, which historical records indicate was non-Vedic. The main religious schools of thought in his society at the time were the nāstika (“heretical” or non-Vedic) schools of Indian philosophy: Ājīvika, Cārvāka, Jainism, and Ajñana. While the Buddha was definitely aware of Vedic Brahmin beliefs and practices, his own ideas seem to have been shaped much more by Carvaka and Jain thought than by Vedic thought. When Gautama Buddha referred to Vedic philosophy, he often did so only to contradict it. For instance, a central idea in Vedic philosophy is atman (the immortal “soul” or “essence” of a person) but the Buddha taught anatman (“no self,” i.e. that the soul does not exist). Buddha rejects the idea of an individual self/soul (atman) and also rejects the idea of an absolute self or universal consciousness (Brahman), directly contradicting the core teachings of Vedic philosophy. In place of the Vedic concepts of the atman (self/soul) and Brahman (universal consciousness), the Buddha posits anatman (non-existence of the soul) and sunyata (emptiness) as the ultimate reality. In this regard, Buddha comes closer to Carvaka philosophy than Vedic (“Hindu”) philosophy. Additionally, the Upanishads speak of Brahma as the creator but the Buddha mocked the proto-Hindu idea of Brahma as a creator god.
In order to justify the identification of Buddha as a reformer of Hinduism, some people point out that many Hindus regard Buddha as an avatar of Vishnu. It is true that some Vaishnavite scriptures identify the Buddha as an avatar of Vishnu. However, they paint him in a very negative light and regard his teachings as heresy. Vishnu is said to have appeared as the Buddha in order to lead bad people and demons astray. He supposedly converted the asuras to Buddhism so that they would all go to hell. Buddha is identified, alongside Kalki, as one of the more malevolent avatars of the Kali Yuga, an avatar coming to condemn and punish bad people (such as atheists) and generally wreak havoc on the world.
Cf. Srimad Bhagavatam 1.3.34 & 2.7.37 & 11.4.22:
“Then, in the beginning of Kali-yuga, the Lord will appear as Lord Buddha, the son of Añjanā, in the province of Gayā, just for the purpose of deluding those who are envious of the faithful theist…. The Lord will bewilder [the atheists’] minds by dressing Himself attractively as Buddha and will preach on subreligious principles…. Propounding speculative philosophy, the Lord, as Buddha, will bewilder the unworthy… And, as Kalki, the Lord will kill all the low-class men…”
Cf. Garuda Purana, ch. 15:
“Then in the Kali age, the lord was born in Kikatas as Buddha. He deluded the asuras and mocked the Vedas.”
Cf. Vishnu Purana 2.8.18–20, where Vishnu, as Buddha, is described as preaching false religion in order to deceive the demons:
“these Daityas [a clan of asuras/demons] were induced by the arch deceiver [Buddha] to deviate from their religious deities and become Buddhists, by his repeated arguments and variously urged persuasions. When they abandoned their own faith, they persuaded others to do the same and the heresy spread and many deserted the practices enjoined by the Vedas and the Laws.”
Proponents of the “Buddha was a Hindu” idea often point to the fact that Hindu deities are mentioned in early Buddhist texts. It’s true that there is mention of various “Hindu” gods in early Buddhist scriptures, but those deities pre-date Hinduism and were already worshipped in India prior to the composition of the Vedas. While deities mentioned by the Buddhist sutras have the same names as modern Hindu deities, they aren’t quite the same gods. Brahma is not a creator god in Buddhism (in fact, he isn’t in early Vedic texts either) and the Buddha mocks the idea of a creator altogether. And Mahakala (Daikoku) is not a fierce destructive entity but a benevolent “lucky buddha.” There is mention of various local deities in the Buddhist texts but they don’t generally have the characteristics of the Hindu gods. And the Buddha doesn’t really think that the gods are important. They may (or may not) exist but they have no ultimate significance. They can’t assist you on your path to enlightenment. Rather than saying that Buddhism developed out of Hinduism, it would be better to say that Buddhism and Hinduism both developed alongside each other, evolving out of various ancient Indian religions and philosophies.
