In countries in the Middle East, Indonesia and many North African countries, the onset of Islamic rulers spread Islam in a rapid manner, eventually wiping off the local religion. However in spite of widespread Muslim rule for several centuries, Islam seemed to have co-existed with Hinduism in India.
How was this possible? Was India lucky that it had tolerant rulers?
I completely disagree with Lennart. Islam has traditionally been expansionist and were big on forced conversions. This is evident if you see the history of India starting with Timur. These forced conversions had been rampart in all of India including the South. An account by Ibn Battuta : South India and Her Muhammadan Invaders
Hinduism is not a religion but a culture. There is no compulsion to attend a particular temple to worship. You can worship at home or even place a stone and imagine it as a god and start worshiping it.
The first wave of Islamic invaders cut 10s and 100s of thousands of 'non-believers' and history suggest many took vows not to eat/drink until they have seen 10,000 hindu heads piled every day (source : Cambridge History of India - Part III). They tried to demolish temples and built Mosques on top of them. This didn't prevent Hindu's worship or follow their culture which they continued practicing privately.
The killing of hundreds of thousands of priest class of hindu did reduced Sanskrit speakers and spread of Sanskrit scholars but gave rise to local vernacular languages (Ram Charit Manas by Tulasidas is simple Hindi translation of complex Ramayana which was in sanskrit)
On some perception that Islam was not traditionally big on forced conversions. I recommend reading Cambridge History of India part 3 covering history of modern day Afghanistan to Bangal between 800 AD to 1500AD. It covers the actual reasons for invading India being spreading Islam buy force. Many Rulers from Delhi sultanate failed to do so and that give reasons for further wave of islamic invaders to try it themselves.
Updated- More on topic : There were resistance from some of the Hindu Kings (Rana's of Mewar) and in down south from Vijaynagar Empire. At the same time there were more invasions by Moughals or Afgans on one of other pretext about conversion of "non-believers" however this gave rise to internal conflicts among Muslim rulers and some time they had to take support of Hindu kings. (e.g. Rana of Mewar supporting cause of Malwa ismalic king against Gujrat Muslim king or in Deccan Bijapur taking help of Vijayanagar to attack Adil Shahi of Ahmadnagar) Or in later part in 1700AD Marattha uprising against Mughals
sources: siyar-ul-mutaakhirin, Cambridge history of India, History Of Marattha (James Grant Duff)
Islam has traditionally not been big on forced conversion. Generally Muslim rulers have been OK with allowing other religions. Especially since non-Muslims often was made to pay extra taxes.
The claim that Islam spread rapidly everywhere else there was Islamic rulers is not a really correct statement from that point of view. It took many hundreds of years there as well. I can't find any support for the position that conversion somehow was slower in India than in most other Islamic empires.
The question of why people convert to Islam has always generated intense feeling. Earlier generations of European scholars believed that conversions to Islam were made at the point of the sword, and that conquered peoples were given the choice of conversion or death. It is now apparent that conversion by force, while not unknown in Muslim countries, was, in fact, rare. Muslim conquerors ordinarily wished to dominate rather than convert, and most conversions to Islam were voluntary. (...) In most cases worldly and spiritual motives for conversion blended together. Moreover, conversion to Islam did not necessarily imply a complete turning from an old to a totally new life. While it entailed the acceptance of new religious beliefs and membership in a new religious community, most converts retained a deep attachment to the cultures and communities from which they came.
-- Hourani, Albert, 2002, A History of the Arab Peoples, Faber & Faber, pg 198
The first few Islamic rulers in India were more focused on land expansion than religious ideals. When the 3rd generation emperor, Akbar, arrived in power, he was famed for promoting religious peace and even creating his own religion Din-i Ilahi, a mix between many different religions. He even married a Hindu princess. After that, most of the Islamic rule in India continued the ideals of their greatest emperor.
More information:
Since your question asks us to concern ourselves specifically with the Middle Ages (5th to the 15th century), I will not venture to comment on the Mughal Dynasty and the shaping of identities in the modern world. However, to ground all of this in present-day observable context, I'd like you to give this a read.
Islam's contact with India can be traced back to the 7th century. For the time-period being observed, and throughout all of history, Islam has come to India through the following two pathways:
Before the Islamic conquests began in earnest with the Raja of Sindh's battle against Muhammad bin Quasim, the Eastern Hemisphere looked something like this:
India appears pretty fragmented at this point. This is because much of her political history has been that of chiefdoms gaining excessive power and breaking away from their kingdoms. There was also great hostility between them.
India, in its modern form, would emerge only in the 13th century, under the Delhi Sultanate. Before that would happen, Muslims were indeed looked at with leeriness by the Hindu population. The Central Asian traveller Al-Biruni attributed it to the fact that the Arabs had, in their conquests, taken several Indians as hostages and there was a general climate of mistrust prevalent among the people.
This wasn't all unsound though, considering that Mahmud of Ghazni and his commanders had plundered and laid waste several Hindu temples that had once boasted great riches.
In 1206, with the coming onto the throne of the Mamluk Dynasty, former kingdoms that were now annexed and under Muslim control, for the first time came under a centralised force. Up until the end of the Medieval Era, Hindus in India easily outnumbered the Muslims. Very early on, it was realised by the Muslim rulers that force wouldn't get them very far.
This is what the Delhi Sultanate looked like at its apex:
Being a highly feudal state, the rulers had no other option but to incorporate Hindus into the administrative system, in light of the fact that most of the land was owned by them. Hindus also had exclusive control over several markets. Also being a theocracy governed by Quranic injunctions, the rulers took it upon themselves to try and bring Islam to as many people as possible and allow people freedom of religion. The kings felt that doing so was executing the will of God. This is why many Islamic holy places (to this day, I believe), guarantee free passage and protection to people of all faiths. Seeing how good a system of checks-and-balances they had established, they had no reason to worry about any province gaining too much power.
Around this time, movements like Sufism also gained ground and Islam as a religion became much more appealing to Hindus, especially those of the lower-castes who were stuck in an establishment that did not permit social mobility.
Another reason why I think there were so few spars between the two religions is because religion isn't the dominant cultural identifier for Hindus. They view themselves of a particular ethnic group and/or region first and foremost.
While it is true that conversion to Islam in India wasn't usually forced, it was almost always implied, as evidenced by this page on Islamic taxes. The rest of the picture isn't as clearly formed either. Many rulers destroyed Hindu temples to provide for materials needed to build mosques and other Islamic sites. Moreover, the frequent change of central power meant that the attitude towards Hindus always remained in a flux, and varied from one emperor to the other.
However, all in all, Islam and Hinduism shared a quite unparalleled cultural bonding in the Middle Ages, the effects of which can be felt to this day.
A couple of ridiculous answers here — such as Islam not being heavy on forced conversions, which is nonsense. The conversion of Persia into Islam is one of the most brutal events in history. Anyone who thinks otherwise is a delusional apologist. The idea that Muslim rulers grew more tolerant is also nonsense. The policies of Aurangzeb in the seventeenth century, of Tipu Sultan in the late eighteenth century and the general popularity of Nizams like the Nizam of Junagadh and Hyderabad in 1947 should be more than enough proof to trash that theory. I can count all the 'secular' Muslim rulers throughout Indian history on one hand with several fingers to spare.
I'm not sure of how organised Indonesian Buddhism was.
But in the case of Sassanid Persia, there was a rigid and highly organised Zoroastrian system present. It was rather easy for Arab invaders to destroy this for a couple of reasons. Fundamentally, the Persians did not see them as the greatest threat, and were more focussed on their civil war. This allowed the Arabs to easily infiltrate most of Persia. From there, they destroyed the holiest cities of the faith like Eshtakhr and killed the priests en masse. Then the Zoroastrians were given a choice, convert or die. A few Zoroastrians did manage to escape, either to Central Asia or to India (Central Asian ones were cleansed in repeated waves by Turkic migrations and later by the Safavids). The Zoroastrians who could not escape were either converted forcibly, enslaved and shipped off elsewhere or were killed.
A genocide of a smaller scale happened in Armenia about three centuries before under the reign of Tiridates III, where Zoroastrians were butchered by the Christians.
In its present state, Hinduism is not a single religion — to assume so is wrong. The word Hindu was first used by Arab invaders to refer to the inhabitants of the plains beyond the Sindhu river (Indus). They called the land 'Hind' and its people Hindus. Therefore, Hinduism is a blanket term for multitude of beliefs held by the people beyond the Sindhu river. There is no central authority to speak for all the Hindus, and neither is there any specific holy book for Hindus like the Avesta for Zoroastrians — there are many, and there is no consensus on which among the many is more important than the rest. This is case in the twenty first century. Back in the eighth century, when the Ummayads first attacked Sindh, it was even less organised.
I believe that the reason why Hinduism as a whole survived repeated invasions by Muslim invaders was because of the lack of organisation, not because of the rulers growing more tolerant with time or any nonsense. Harder to break something if you don't know what to destroy — not that anyone can ever accuse them of not trying hard enough.
There are plenty of records which show that Islamic rulers did not, in fact, grow "more tolerant" as time passed. Aurangzeb, the last of the Mughals to rule over a substantial part of the Indian subcontinent reintroduced the Jaziya tax, and plundered several temples throughout his reign. Tipu Sultan, a hundred years later, started massive pogroms which resulted in some areas on the south western coast becoming dominated by Muslims — a demographic that persists to this day.
