(This is a slightly overwhelming post. I would be happy if anybody could answer even part of my questions)
Buddhism originated from the mainland Indian subcontinent (specifically the Eastern Indo-Gangetic plain in Bihar), but went all but extinct with most Buddhists in India (0.7% of the population) being recent converts to escape the Hindu caste system. I understand why Buddhism went extinct in India (there have already been multiple posts about Buddhist decline in India), but then why did Buddhism not only survive, but thrive in Sri Lanka? Considering how heavily Sri Lankan history, culture, politics, architecture and national identity stem from and are intertwined in Theravada Buddhism, it would be a surprise to most people when crossing right across the Mannar strait to India, Buddhism is little more than a fuzzy historical memory.
So what was different in Sri Lanka (from India) that allowed Buddhism to survive intact and flourish? Buddhism (especially Theravada Buddhism) arguably would be very different or even non-existent were it not for Sri Lankan monks writing down Buddhist scriptures such as the Tripitaka, and recording much of Buddhist history and thought.
3 reasons I've often heard about the decline of Buddhism in India (I'm going to be comparing these to Sri Lanka) are that:
a) The rural (and overwhelming majority) of the population remained Hindu- if this were the case in India, why did it not stay the same in Sri Lanka?
b) That Indian kings began to support Hinduism and stopped supporting Buddhism- what then made the Sinhalese kings continue supporting Buddhism, even in the face of repeated Tamil Hindu invasions?
c) That Buddhist monks lost debates to 'Hindu' (I put this in quotation marks as they wouldn't have seen themselves as part of an unified religion in opposition to Buddhism) revivalists, scholars, and priests. If this was the case, why did it not happen in Sri Lanka? Was it not visited by these Hindu priests? Did the Theravada Buddhists win the debate in Sri Lanka?
Thanks in advance if you can answer any part of my question (s).
There are many similar questions with fantastic answers, which I shall link below. I can answer part of your question briefly here.
The decline of Buddhism in India can be attributed to two primary reasons, the Muslim invasions in the 12th century, and the "stagnation" of Buddhism at this time. "Stagnation" is perhaps too negative a term. However, what I mean by this is that Buddhism in the Indian Subcontinent by the 12th century had become less energetic, proselytizing, and innovative.
Buddhism historically flourishes when it receives the patronage of the ruling class. The relationship in the Sangha between the ordained and the lay community is that of mutual benefit, the monks offer spiritual services, guidance, and teaching, while the lay community ensures its survival with financial and material support. Ordained monks are prohibited from growing food and practicing trade, thus, they rely heavily on sponsors from their local community, and on a wider scale, lords and kings.
Just as important is understanding how Buddhism originated in its native environment. Buddhism originated and evolved as a reaction to early Vedic traditions/Brahmanism. Buddhism rejected the caste system, as well as the notion of nonself, against the Vedic understanding that the self is in unison to Brahman, part of a universal soul. Early Buddhism also deemphasized rituals and worship of gods, the former which was only accessible to higher castes.
With the Muslim invasions, not only were Buddhist institutions sacked and monks massacred, taxation and seizing of lands further inhibited the communities. As elite patronage was no longer possible, the lay community slowly died out. Jainism and Brahmanism survived due to Jainism's more demanding and active relationship between the ordained and laity, and brahmins are more involved in social, political, and legal matters. Buddhism was too inward looking, considering the renunciatory customs of the ordained. What I mean by stagnation is not laziness, idleness, or corruption. There was just no creative outburst and development like in the first and sixth centuries. Many elements of Buddhism became incorporated and mixed into Brahmanism/Hinduism. The Buddha and Buddhist deities were folded into the Hindu Pantheon. Iconography, mythology, and vocabulary were already similar and shared to begin with, they gradually were assimilated in full. Finally, many monks relocated to Nepal, Tibet, or Southeast Asia.
In Sri Lanka, Buddhism was not flourishing continuously. Sri Lanka saw just as many divisive and tumultuous periods as India did, with invasions and warring kings and states. Figures such as Kalinga Magha controlled the island's north and devastated the Buddhist community. Just as in India, foreign invasions, pirates, and civil strife prevented financial support for the Sangha. Many times the monks were so few that envoys were sent to Burma or Thailand to recieve more numbers and texts. Portuguese invasions in the sixteenth century also persecuted Buddhists in favor of Catholicism, and southern kings such as Rajasinha I further razed Buddhism temples. It was British rule however that saw a true revival of Buddhism. Initially led by Henry Steel Olcott through his Theosophical movement, other figures such as Anagarika Dharmapala heavily embraced the historical Buddhist identity and promoted its revitalization.
References
Questions of this nature are quite frequent on this sub. Here are some further or related readings:
a) The rural (and overwhelming majority) of the population remained Hindu- if this were the case in India, why did it not stay the same in Sri Lanka?
Sri Lanka- in terms of the Buddhist/Sinhala population- does not seem to have had the same degree of Hindu/Brahmanist indigenous influence that mainland India had. While the stories of the origins of the Sinhala are heavily mythologized, they all essentially equate to the idea of a northern Indian people (who may not have been Buddhist yet, but may have been part of the non-Vedic cultures that existed alongside Vedic India) colonizing an island that was mostly inhabited by animist indigenes. Events recorded in the mainland Buddhist Vinaya (monastic law) tradition suggests that while mainland Buddhism developed alongside well-established Brahmanist/Hindu traditions, in Sri Lanka Hinduism wasn't the dominant local ideology, except among immigrants from Southern India.
b) That Indian kings began to support Hinduism and stopped supporting Buddhism- what then made the Sinhalese kings continue supporting Buddhism, even in the face of repeated Tamil Hindu invasions?
Sinhala kings were actively engaged in trying to maintain their independence from mainland India, and even in certain eras trying to assert control over regions of India and be a "player" in the contest for control of regions of India. Mahavihara Buddhism (Theravada) was largely under the control or influence of Sinhala kings, whereas other schools of Buddhism and Hindu thought were either well established enough to exert their own control, or were under the control of Hindu or Buddhist rivals in the mainland. The independence of Sinhala Buddhism was part of how Sri Lanka exerted an independent identity and maintained its uniqueness from southern India and other regional rivals. These kings were directly threatened by the Tamil invasions, and Sinhala Buddhism provided an ideological support to their struggles to repel the southern India invaders- it allowed them to say to the public "we are standing up for your beliefs, not just the dominance of one oligarchy over another".
c) That Buddhist monks lost debates to 'Hindu' (I put this in quotation marks as they wouldn't have seen themselves as part of an unified religion in opposition to Buddhism) revivalists, scholars, and priests. If this was the case, why did it not happen in Sri Lanka? Was it not visited by these Hindu priests? Did the Theravada Buddhists win the debate in Sri Lanka?
Part of what went on was less "losing debates", than the dynamism and popular support following the areas where it seemed that new innovations were happening. Buddhist universities, for instance, also taught Hindu thought and Sanskrit in order to enable students to engage in debates with Hindu thinkers and compose prose and verse in ways that were regarded as "civilized" by the wider cosmopolis. In many situations, the points that separate Buddhist thought from Hindu such as denial of the eternal self or Atman and the non-primacy of deities were 1) beyond the reasoning of the average illiterate lay person, and 2) softened over time, either by mutual influence or the evolution of Buddhist practice. If you are making offerings and reciting hymns to the Buddha and the Bodhisattvas but there is nothing in your religion that says you can't also make offerings to other deities, then for the average lay person the distinction between being a Buddhist who is a client and donor to a monk vs. being a Hindu who is a client and donor to a priest or Hindu monk begins to be blurred.
To a certain extent, you have to recognize that the average person is not committed to ideological positions but is instead a consumer of religion and patron of various shrines and priests. If the Hindus claim to have a magical verse that protects you from lightning, then the Buddhists have to have one too. We see that Buddhism over time begins to adopt these aspects of popular religion that, whatever their origin, tend to blur the distinction between Buddhism as a unique spiritual path vs. just another of the multiple gods that you can patronize for protection, good fortune, etc.
So in Sri Lanka you have a socio-linguistic group (the Sinhala) that recognizes Hinduism as being part of a foreign power that potentially threatens their own dominion and independence. In the mainland, you have Buddhists and Hindus in what is already a religiously plural environment actively cribbing practices and doctrines from one another in order to gain popular and elite support. You have institutional structures that are blurring the distinctions as well by teaching a more cosmopolitan vision of what it means to be an educated monk or priest, and a lot of illiterate and semi-literate lay people who are only dimly aware of the distinctions between a Buddha and a god to begin with.
Certainly the isolation of Sri Lanka from the Muslim raiders in the north plays a part, but it's not as though the Buddhist institutions of Sri Lanka were perfectly preserved. The Portuguese destroyed most of the temples and libraries, and killed or drove out the monks to the point that there were no longer enough monks to conduct new ordinations. The difference is that there wasn't an indigenous non-Buddhist tradition that could absorb the remnants of Buddhism in Sinhala society. The loss of the mainland Buddhist institutions probably helped the Mahaviharans more than it hurt, because it meant that there were no longer any rivals for Buddhist influence in Sri Lanka, and no mainland Buddhist temples that heterodox monks could decamp to, or send missionaries from.