Shankara produced a classical method for Hinduism, as new form of Brahmanism. The castes were back, the old order was back, Buddhism as ideological enemy was beaten.

Despite this, here is what we read in the Bhāgavata Purāṇa, the Divine-Eternal tales of Supreme God, 18,000 verses written down in the 9th or 10th century CE (but existing before in the oral form) :

“Neither Yoga nor Samkhya nor dharma nor the study of the Vedas, nor religious austerity or abandonment captivates He (so thoroughly) as intense devotion. I, the beloved Self of the righteous, can be captured (only) through exclusive devotion with reverence.

Devotion concentrated on He absolves even the Svapakas (who cook and eat the flesh of dogs) from stigma attached to their birth.”

Indeed, Hinduism triumphed but only formally. Shankara's perspective was not followed, because it was too rigid, too uniform and not adapted to the multiple local situations requiring the integration of the specific local gods.

Therefore we find, in ideological competition with Shankara, Madhvacharya (1199–1278 CE) and Ramanuja (1017–1137 CE), who developed a non-dualist system.

Whereas Shankara called to join the “one”, because everything else was an illusion, exactly like Plato and its allegory of the cave, Madhvacharya and Ramanuja promoted a traditional dualist system.

According to them, there was the universe and there was god, and humans had to praise God as lord of the universe.

This is the field of Vaishnavism, the cult of Vishnu; but we also find, in parallel to this and at the same period, “Shivaism” i.e. the cult of Shiva. Shivaism was very common though not as powerful, and ideologically stronger on ancient India's limits, Kashmir for instance; it was mostly connected with magical and mystical trends which still existed in this period.

The celebration of Shiva went for example with the cult of the lingam, the phallus as symbol of energy; Shiva was in fact a pre-vedic god, which was integrated in the vedas through an assimilation with the vedic god called Rudra, a process finalized in the Shvetashvatara Upanishad.

Vaishnavism was the popular form of Hinduism; still today the “non dualist” system of Shankara is highly considered but far from being really popular. Vaishnavism was indeed so popular that it is linked to the movement called the “bhakti” (“portion”, “share”, “attachment”, “devotion”, “fondness for”, “devotion to”, etc.).

As we saw in the quote above, the bhakti movement accepted the Hinduist theology, but rejected more or less the castes, and even the vedic rituals, in the name of a direct connection to the godhead, through singing, praying, devotion, etc.

The most amazing phenomenon is that the bhakti movement, despite its positions, was eventually integrated into Hinduism, living peacefully alongside the orthodox tradition.

How could a popular movement of devotion to the unique god for all coexist with orthodox Hinduism?

It is because the bhakti movement was the form taken by the historical advance of the medieval feudal ideology and social basis. Orthodox hinduism was still the ideology of the villages, but wherever traditions got entangled in complex situations, then we find this bhakti approach, which is in fact an equivalent of early medieval Christianism in Europe.

Orthodox Hinduism had to accept this to protect its own traditions from the mass movements, but also because the bhakti movement was an ideological weapon for mass mobilization to stop the spreading of Islam.

In fact, if in orthodox Hinduism it is the priests who are in a central position, in the bhakti approach it is the whole society i.e. the local rulers; it was the supreme god which was venerated, either as himself or in the form of another sub-god or goddess, or even in the form of a teacher, a “guru”.

Whereas Shankara called to understand in a rather rational way one's own vacuity to join the supreme ego, the bhakti explained that love for god permitted to merge with him, in a mystical perspective, because we live in the last yuga, the last cycle of the period of 4.1 to 8.2 billion years when the universe is destroyed and created again.

The influence of Islamic sufism had an obvious role in the individual relationship with god, whereas previously god was considered as an entity equivalent to the universe and thus remote and on a level out of reach of all but pure souls.

Now, anybody could “merge” with god through total devotion, prayers, collective chants, etc. What is called often the “Hare Krishna” movement in Europe and the US is based on the Bhakti tradition, and the goal of the follower is ressemble Radha, the woman loved by the supreme god Krishna.

The Bhakti movement does not call for annihilation of the ego, but for an encounter with the supreme; its concurrent was not Buddhism, but Islam, with the appearance of feudalism in the Indian situation.

Let's quote here the Indian mathematician Damodar Dharmananda Kosambi (1907-1966), who was also a great historian. This shows why his understanding of dialectical materialism is of such great value :

“Dialectical materialism holds that matter is primeval and the properties of matter are inexhaustible. Mind is an aspect of matter being a function of the brain, ideas, therefore, are not primary phenomena, but rather the reflection of material processes and changes upon human consciousness, which is itself a material process.

Therefore, ideas are formed ultimately out of human experience.

Matter is not inert, but in a constant state of interaction and change; it is a complex of processes rather than an aggregate of things. In every stage, there resides an inherent quality of change, an “inner contradiction”; which leads eventually to a negation (not necessarily unique) of that stage or condition.” (Exasperating Essays: Exercises in Dialectical Method).

Let's quote here his work Social and economical aspects of the Bhagavad-gītā:

“However, the Gita did contain one innovation which precisely fitted the needs of a later period: bhakti, personal devotion. To whoever composed that document, bhakti was the justification, the one way of deriving all views from a single divine source.

As we have seen from the demand for the quite insipid Anu-Gita sequel, this did not suffice in its own day. But with the end of the great centralized personal empires in sight, Harsa's being the last- the new state had to be feudal from top to bottom.

The essence of fully developed feudalism is the chain of personal loyalty which binds retainer to chief, tenant to lord and baron to king or emperor.

Not loyalty in the abstract but with a secure foundation in the means and relations of production: land ownership, military service, tax-collection and the conversion of local produce into commodities through the magnates. This system was certainly not possible before the end of the 6th century AD.

The key word is samanta which till 532 at last meant 'neighbouring ruler' and by 592 AD had come to mean feudal baron. The new barons were personally responsible to the king, and part of a tax-gathering mechanism.

The Manusmrti king, for example, h

ad no samantas; he had to administer everything himself, directly or through agents without independent status.

The further 

development of feudalism 'from below' meant a class of people at the village level who had special rights over the land (whether of cultivation, occupation, or hereditary ownership) and performed special armed service as well as service in tax-collection.

To hold this type of society and its state together, the best religion is one which emphasizes the role of bhakti, personal faith, even though the object of devotion may have clearly visible flaws.”

With bhakti, Hinduism found a way to promote god in general, through a personal devotion which suppressed any rational perspective; India could sink in feudalism with this religion existing alongside orthodox Hinduism as expression of the continuation of ancient India's village system.