Not really my area of expertise, but aren't "people of the book" such as Christians or Jews afforded some tolerance? In some places being Christian in an Islamic country was better than living in most of medieval Europe, which served those nations well as I recall.
This depends upon what you term tolerance; these discussions on Islam can and never should be framed in the sense of Europe and its concept of religion which is anathema in some ways to that of Islam. Our societies are in opposition in terms and conceptions of words lost in translation. One of the most misunderstood when Islam is applied to the West, is the idea of 'tolerance' or freedom of religion, each of which do not exist in Islamic culture. There is no such thing as tolerance, only those to whom as I have explained are under fealty to Islam and or otherwise totally and abjectly subject to the power of Islam and thus by their submission totally and completely, give praises to Allah and blessings to the Muslim by the fact that he does as Allah commands and subjugates those who disbelieve.
However, these issues can be complex and I will give a short description of the status of ahl al-Dhimma and other religions with the Abbasid period. The Abbasid period is by far the most important as nearly all precedents in Islamic fiqh (jurisprudence) are set during the Abbasid period and all schools of mainstream Sunni fiqh are developed during the Abbasid hegemony.
-To begin, the status of Dhimma is the term, 'one under a truce, with the stipulation of taxation,' such a term cannot ever be separated from the pre-Islamic (Jahiliyyah) Arab contract laws and hakm (ruling). Arab contract law was based near entirely upon the concept of security pacts and those agreements from which one was considered protected, but also submissive. Arabs prior to Islam used these ideals as the height of nobility and the traits with which defined Arab peoples and separated them from Jews, who used Mosaic contract laws, whose stipulations differed from that of the Arabs. These security pacts were such that one group or an individual was kept under another by the basis that he had accepted a submissive role to another stronger person and was thus defended by this person and in most cases the tribe from which the powerful person derived, also was defenders of such a pact. These security pacts were of such great importance to Arab nobility, that Muhammad ibn Abdallah by virtue of being in a security pact under and a member of a noble branch of the Quryaish was not to be harmed until a later time; this was even so by Abu Sufyan who when asked regarding the status of Muhammad's birth, he answered that Muhammad was from amongst the most eloquent of the Arabs and his pact with his kin is strong and thus, murder of him was not honorable.
From this background, we understand the Christians and Jews as those people who have been conquered and submissive to the Muslim religion. Their status is one of two in Islamic fiqh of the Abbasid period:
>Dar al-Harb: Those whom the Muslim are in a perpetual state of war with and are those states whom are in a state of fitnah or mischief that places them in opposition to the Muslim whose single goal in existence is the eradication of fitnah both in their individual personage through internal Jihad and on the externals of the dar al-Islam by way of physical jihad through dawaa (missionary work, if you like) or by harb/war.
>Dar al-Hudna: Those whom the Muslim use or have a truce with. Each of the two are similar and interlocked, in the sense that a Dhimmi has a pact with the Muslim and thus benefits the Muslim by submitting and by paying the jizya. However, external states or groupings are also considered to be ones within dar al-Hudna. Thus, a great example are states opposing the dar al-Islam who sign a temporary treaty of peace or alliance of sorts. As well, groups of people who have a certain usage that can be utilized by the Muslim in some way, can be placed into this pact. A major case of that form of pact, is the Abbasid agreements with the Paulicians, who were not eligible for the status of Dhimmi and thus could not pay the Jizya; however, their martial skill and groupings along the Byzantine border, allowed them to have a use with which the Muslim could exploit and thus, it is said, they paid a jizya in the form of lost blood.
Jews and Christians were thus, not tolerated by the Abbasid, they simply were allowed to exist in the sense that the groups were already submissive to the Muslim. All forms of public religion were specifically banned by all forms of fiqh. A person for instance who attempted to spread his religion to a Muslim was one who was prescribed death in the hadood. Jewish and Christian women were required to wear the veil and males of those faiths were also required to follow roughly Islamic dress, thus no tight wearings, offensive idols upon the neck, head, ears, ankles, wrists, etc...There are many such requirements however, that for the sake of brevity, I will not mention.
Other faiths of course depended. Many where rural to the point of lacking the interest which would of lured the authorities into persecution. Others however, were persecuted in a very powerful sense.
Manichaens: Manichaens for instance, whilst at times these Manichaens practiced dissimulation and contributed to the city of Baghdad, it is more the common narrative that Manichaens were ruthlessly purged from the Iraqi landscape and removed completely by the 10th century in Baghdad. Manichaens most principally were hated by various Caliphs and their persecution was pursued personally by the Abbasid elites and by the Mu'Tazilah who saw the sect as irrational.
Khawarij: Khawarij were obviously purged in any case in which a community openly claimed such views. Khawarij were punished in many brutal ways, during the first two Khilafah. For instance, Ali ibn Talib burned Khawarij rebels at the stake (there is disagreement on this; Shafi'i believe that he literally burned Khawarij at the stake and also burned the followers of Abdallah ibn Saba, and thus Shafi'i are of the opinion that burning is a permissible form of execution [see ISIS or punishments levied by various Shafi'i states of the past, which at times, included burning], however, the other three madhahb claim that he did burn them, but was incorrect in doing so; against both positions, Shi'i faqih [of all kinds] claim that Ali ibn Talib did not burn anyone and instead believe the term 'smoked them' referred to a torture by which he set a fire and covered them in smoke so that they choke from the fumes). In addition, Khawarij were so feared that the simple mention of their existence in a region, would lead to the Abbasid authorities saturating the entire region in some form of examining to capture and execute them.
Shi'i: Viciously persecuted and attacked by the Abbasid authorities after the 820s at least. Many Alids were executed by the Abbasid authorities and at many occasions, the tomb of Husayn was off limits and all pilgrmages to Karbala, Kufa and Najaf, were prohibited. There is a clear reason for which the Shi'i Imamiyyah gave taqiyyah as a blessing unto the Shi'i; a blessing and tactic with which the Shi'i may use to simply survive the persecution of the regimes of the day. Further, even during the reign of Hajaj ibn Yusf, you see Umayyad officials who killed Alids en masse and murdered large numbers of Shi'i peasants unfortunate enough to be targeted as subversive. This is not even mentioning the Mihna (inquisition of 830s-870s), which aimed to not only rectify Sunni Ulema, but to uproot Shi'i Islam from Iraq.
Zunbils: The famous people of the Zwambinar or Zabul, who revered the Sun, Zun, as the ultimate god. They were once conquered, I presume, forcibly converted to Islam or sold into slavery. Further, many were killed as they rushed to defend their idols covered in jewels and were cut down by the Abbasid/Saffarid armies. The Abbasid period prescription in the case of the Zunbils, was either convert or death, this is a clearcut issue.
Buddhists: Buddhists of Central Asia and Afghanistan were seemingly forcibly converted by the advancing Muslim armies or fled en masse to different areas. It should be noted, that by all accounts, any idol of Buddha or other pictures of relevance, were burned and desecrated by the Umayyad-Abbasid armies and eventually by the Islamized Turkic armies. The exception being the large statues of Buddha in Bamiyan (eventually destroyed by al-Qayda; despite the resentment of the local Taliban against the destruction [one should note the level to which the Taliban were an ethnic fighting force with high amounts of syncretic beliefs]) with which the Abbasid forces not familiar with siege technology, had no way of destroying (the same is true of the Pyramids). We have no evidence further of Hindu/Buddhist temples in Afghanistan, other than caves where Buddhist monks likely survived after the conquests and period of struggle. I would personally assume that they were all destroyed as idols.
Zoroastrians: This group was most certainly persecuted acutely. Abbasid period opinions of the day where most greatly surmised in the trial of al-Afshin and Mayzar al-Qarvaniyya. (this is partly a reply to
@Dfront21 )
Al-Afshin, the most talented general of his time in the Abbasid period and the famous general who defeated the Byzantine emperor at the battle of Anzen (838) and was considered the personal favorite of Caliph al-Mu'Tasim. Mayzar al-Qarvaniyya, was a Tabaristani/Dalaimite feudal lord who converted to Islam to gain protection from the Abbasid state who guaranteed the protection of his estates (at the time, his state was the last Sassanid holdout in Iran, due to the impregnable fortresses there).
Al-Ashin himself, was a convert. His feudal estate was the entirety of Sogdia and Ferghana; what I deduce from the sources, at around the early 790s, the Zoroastrian/Buddhist rulers in Ferghana-Sogdia engaged each other in a civil war of sorts, over the status of their lands. The conflict seems to be one of whether to remain independent and suffer the increasing attacks from Turkic tribes to the north, previously held in bay, wait for a power to capture the region (such as Tang and not the Abbasid) or submit to the Abbasid by converting to Islam and joining the Abbasid hegemony. After said conflict, the lords of Ferghana announced their intention to convert to Islam and submit to the Abbasid throne. Al-Afshin was the ruler of this land at the onset of the 830s and commanded some of the most exquisite cavalry divisions in the Abbasid hegemony. Due to this, and his perceived crypto-Zoroastrian traits, he was commanded to wage war against the Zoroastrian rebel, Babak Khurramiyyah. Al-Afshin, proceeded to gain victory and thus proved himself by later his performance in the Amorium campaign and in a short time, solidified himself as the greatest commander in all the Abbasid hegemony.
However; while al-Afshin was away in battle against Byzantium, Mayzar al-Qarvaniyya unleashed a scheme wherein he would capture Abdallah al-Tahir (the governor of Fars/Iran) and gain control. With this attempt, he reverted to Zoroastrianism and planned his assault. However, his bodyguard betrayed him before he could implement his plans and he was taken to Abdallah al-Tahi who whipped him and humiliated him and took him to Baghdad. Once placed in interrogation, Mayzar revealed that his accomplice in the scheme to capture Iran was that of al-Afshin and explained in detail his plans and produced written records supposedly penned by al-Afshin (possibly forged by Mayzar to frame al-Afshin). Al-Afshin thus upon his victorious return was captured by Abbasid guards and taken to be interrogated. Al-Mu'Tasim however, halted the process and demanded that al-Afshin be given a trial and swore that he himself could attest to the fact that al-Afshin was Muslim.
The trial would become infamous in Islamic history, two feudal lords of the highest class, accused of being Zoroastrian reverts and as schemers against Islam. Mayzar admitted his wrong doing and in many ways sought to be executed; however, this position of his made him an asset in seeing that al-Afshin was found guilty. One by one, the evidence stacked opposed to al-Afshin:
-First, the evidence of the ransacked Afshin estate and the procuring of his personal items. All of which revealed idols to various gods and emblems that exhibited a status of having a traditional faith or Zoroastrianism. The evidence from his estate included altars and other priceless memorabilia. These items dammed him almost immediately in the eyes of the ulema in Baghdad, who had ruled that such items must be destroyed and that a Muslim carrying such was liable to the penalty of death. Further, all forms of idols, whether owned by a Muslim or non Muslim must be destroyed and those who worship them punished, unless they are among the Dhimmi (not Zoroastrians).
-Second, Muslim witnesses arrived and made statements that al-Afshin had preserved Zoroastrian temples and religious sites. One example was where al-Afshin hindered and exiled a group of Muslim muhjahid from destroying a Zoroastrian temple and other sites in his lands. Within Abbasid fiqh of the time, it was allowed and obligatory for either an individual, group or the state to destroy all offending idols, if possible. A ruler who hindered such destruction was seen as one of the Taghoot (transgressors) and in many cases carried the penalty of death. Abbasid investigation thus agreed and concurred that yes, despite rulings on the temples and sites of Ferghana, they had been maintained and not destroyed; this piece of evidence completely damned al-Afshin in the eyes of the time.
>What this shows is that the zeitgeist of the time, was to abuse Zoroastrians completely by destroying their religious sites.
In the end, al-Afshin was sealed away in a white tower where he was starved to death. Mayzar al-Qarvaniyya was taken into the public square and tortured then strapped to a pole and skinned alive before he was then beheaded. He, al-Afshin and Babak Khurramiyyah were both skinned and their skin, head and bodies were denied burial and displayed in public.
Also do note, that until the Buyyid period, the Zoroastrians were never under Shi'i faqih of any kind. Thus, the Shi'i opinion on their status is irrelevant. We are discussing the Abbasid-Umayyad period, those who principally dealt with Zoroastrians.