Both Buddhism and Jainism were driven by their animus against Brahmanism and the Caste system. But both did not acknowledge a deity. Thus, though the moral tone of both was elevated, they both lacked the emotional warmth that religions with a connection to a deity provide to their votaries. And this was the chink in the armour of both Buddhism and Jainism. Both these religions had overcome Brahmanism and held sway over a majority of the population of India, especially after the reign of Asoka, and right up to the 9nth century.
The first pushback against these religions was kinetic in nature and was led by Pushyamitra Shunga who was the commander in chief of Brihadratha Maurya, the last Mauryan king, whom he murdered to establish his own dynasty. Being a Brahmin he was especially vested in the defeat of Buddhism. At the very outset of his rule, he set about persecuting Buddhists and destroying their monasteries.
It is about the nineth century AD that Hindu renaissance began with an intellectual cum spiritual effort against both these religions, and this started in the South. In the van were two saintly orders called the Alvars and the Adyars,[ championing the cause of Vishnu and Shiva respectively as the One Supreme God] and both struck at the weakest point of Buddhism and Jainism–the absence of emotional content in their belief system.
They relaxed the rigours of the caste system and allowed the lower order to join in worship, which was primarily the singing of hymns of love and praise of God, and theoretically at least paid lip service to equality of mankind. Millions not understanding the high moral tone of Jainism and Buddhism, gravitated to the warm emotional appeal of the new Hinduism. In time Jainism was submerged in this tide while Buddhism was washed away to distant shores away from India.
But just then Hinduism was confronted by the challenge of Islam.
Shankara [c. AD 800] a Malabar Brahmin, began a movement to refute Buddhist and Jain “heresies”, and started a movement of renaissance and reform in Hinduism. This became of decisive importance to Hinduism’s attitude to Islam.
Shankara was a metaphysician and he essentially provided Hinduism’s protagonists with debating points. He did little more than this to push forward the mass movement begun by the Alvars and Adyars.
That base established by Shankara was built upon by Ramanuja, who disagreed with Shankara’s reliance purely on logic and advocated the use of Bhakti [devotion] which was recommended centuries earlier by the Gita. He traveled over large parts of North India, including Kashmir and left large numbers of disciples there.
The cult of Bhagti was popularized in Northern India most effectively by Ramananda. He allowed lower caste Hindus and Muslims to join him in worship and to become his followers. His disciple Bhagat Kabir became the greatest exponent of the Bhakti cause, after whom this movement is named.
The Beliefs and Teachings of the Bhagtis were:
-One God, who was indescribable, but was the ONLY reality. All the rest was Maya [illusion]
-The best way to serve God was by absolute submission to His will.
-The best way to approach him was through meditation, the chanting of mantras and the singing of hymns.
-This was best achieved by spiritual guidance of a Guru.
There was a fair degree of overlap between the teachings of Muslim Sufis and the Bhagtis and both attracted large number of lower caste Hindus to their cause.
Baba Nanak’s teachings were an amalgam of Bhagtis and Sufis.
But it remains an amazing fact though, that despite all their effort, Buddhists, Jains, Bhagtis, Sufis, and Sikhs, tried to uproot the caste system through the centuries, but failed.