Christian and Jewish fundamentalism have certainly existed over time, not least during the periods of the Crusades with respect to the former. It is important, however, to draw distinctions between "fundamentalism" and "extremism." Fundamentalism may simply imply a more orthodox view of religious practice such as exists among Hasidic and, to a much lesser extent, Orthodox Jews. While Hasidic Jews are militant in defense of their freedom to practice their religion as they see fit, and are intolerant of those who practice less-orthodox versions of Judaism, they do not pose a threat to those who respect their practices, which are peaceful, and do not defile their Sabbath. Other than that, Jewish extremists have never represented the Jewish people in any meaningful way. On the contrary, such extremists as defended Masada, and who assassinated Prime Minister Yizhak Rabin almost two thousand years later, have never represented more than a tiny fraction of followers of Judaism.
Christian fundamentalism today is also a matter of more orthodox interpretations of Scripture but, again, while intolerant of other interpretations, and while representing a politically-potent perspective, is not the threat to other communities that resides among religious extremists such as exist among Hindus in India and Muslims throughout the Islamic world, and herein lies the crux of the matter. Christian Europe has accepted millions of Muslim immigrants since the post-World War II period, and there have been isolated instances of anti-Muslim extremism among disaffected Christian youth, like Skinheads and neo-Nazis, but widespread attacks sanctioned by mainstream political parties have not been the norm. Far-right political parties like those in France and Denmark have existed on the fringes of those countries' respective political scenes, but have not been representative of the majority of Christian voters. In short, fundamentalism and extremism have been marginalized and usually ostracized, remembrances of the intolerance that led to the Holocaust remaining strong in many minds.
Islamic fundamentalism, in contrast, exists in many regions of the Islamic world. Fundamentalism, however, is not necessarily the threat to peaceable communities that the word implies. Rather, it is Islamic extremism that poses this threat. As with other religions, there are degrees of orthodoxy with respect to the practice of Islam. The most populous Islamic nation in the world is the Southeast Asian archipelago of Indonesia, which is known for its very tolerant and moderate brand of Islam. Saudi Arabia, the land of the two holiest sites in Islam, at Mecca and Medina, is fundamentalist and extremist in that it practices an intolerant form of Islam called Wahhabism, after the 18th century Muslim cleric Muhammad ibn ʿAbd al-Wahhab, who operated jointly with Muhammad ibn Saud in forging the kingdom that would become Saudi Arabia. The arrangement between the al Saud family and the followers of Wahhab allowed both for the political domination of the House of Saud and the religious domination of Wahhabism, which has bred extremism.
Extremists have existed within the Christian and Jewish faiths. In the modern era, however, it is Islam that is most personified by the face of intolerance, and that is unfortunate. As noted, over 200 million Muslims live peaceful lives in Indonesia, with millions more in the nearby nation of Malaysia. Throughout the Islamic Maghreb across Northern Africa and within the Levant of Lebanon, many more were content to live peaceably with their neighbors until stirred up by extremist clergy and politicians. The Qu'ran, the Muslim holy book, is replete with examples of intolerant and even murderous instructions for followers, and is quite anti-Semitic, but most Muslims viewed other faiths as legitimate expressions of fealty to God. What is troubling, however, is the degree to which many millions of Muslims are willing to accept the most horrific and libelous characterizations of practitioners of other religions, especially Judaism. Scratch beneath the surface of some of even the most tolerant of Muslim communities, and an intense hatred for Jews is uncovered. Jewish identification with the land of Israel is anathema to Muslims, despite the Jewish presence there hundreds of years before the birth of Mohammad. Even if the Palestinian issue were satisfactorily resolved, such sentiments would continue to be widespread among tens of millions of Muslims.
This educator has traveled through the Middle East, including to Saudi Arabia, Israel, and the Palestinian territories, and has traveled to Indonesia. The disdain for Judaism is profound and truly discouraging, and it is the extent and depth of such feelings that sets Islam aside from Christianity and Judaism in today's world. Islamic extremism is a far more powerful force in Islamic regions than Christian and Jewish extremism are anywhere in the world. Examples of Christian and Jewish fundamentalists and extremists can certainly be found, but such examples are the exception. In much of the Middle East, it is more the rule. It is important to remember that, following the events of the so-called "Arab Spring," Islamic fundamentalists and extremists enjoyed over 70 percent of the popular vote in Egypt's elections. That is not encouraging for the most densely populated Arab nation, although lingering anger towards the now-ousted government of President Hosni Mubarak certainly played a role in that outcome. The support Islamic extremists continue to enjoy is far more widespread than is the case with other faiths, again with the exception of Hindu fundamentalism in India. The Jewish Defense League, an extremist but not fundamentalist terrorist organization active in the United States during the late 20th century was an outcast among the Jewish communities of the nation, rejected for its extremism. Organizations like al Qaeda, the Taliban, and the Islamic State can continue to recruit over 1,000 new militants every month precisely because the views represented by these organizations, if not always the tactics, are embraced by many Muslims.