There is plenty in both the theology of Islam and the behavior of Muslims which outsiders can legitimately criticize or disagree with. Any serious, sustained critique, however, must be based on what Muslims actually believe and this in turn requires understanding just how diverse Islam is. Comments from both Muslims and critics can give the impression that Islam is a single, united, monolithic religion but this is false. There's more to Islam than most seem to realize, even among atheists.
There may not be quite as much diversity in Islam as in Christianity, but as with Christianity in the West there is a division between two significant players. In Western Christianity, the division is between Protestantism and Catholicism. In Islam, the major division is between Sunni and Shia Muslims. Most critics recognize that it would make no sense to criticize Protestants by complaining about the pope, but not all realize that similar mistakes can be made with Islam as well.
Sunni vs. Shia Islam
Most statements about Islam apply to Sunni Islam, which represents the vast majority of Muslims. Although the differences between Sunni Islam and the various Shi'ite sects started out as political, the distinction between the two groups has gradually become more and more theological as well. Shia Muslims continue to hold the same fundamental beliefs of other Muslims, with the principle addition being that they also believe in an imamate, which is the distinctive institution of Shia Islam. The doctrine of the imamate was not fully developed until the 10th century and other dogmas developed still later.
Sunni Muslims view the caliph as a temporal leader only and consider an imam to be a prayer leader, but for the Shia the historic caliphs were merely de facto rulers while the rightful and true leadership continued to be passed along through a sort of apostolic succession of Muhammad's descendants, the Imams (when capitalized, Imam refers to the Shia descendant of the House of Ali). The conflict between Sunni and Shia Islam can thus be said to be fundamentally about the nature of religious authority: is it conferred and transmitted through rational, legal institutions or does it include a charismatic, mystical element?
Religious Authority: Rational & Legal
In principle, Sunni Muslims' relationship with God is direct and is not mediated by anything like a priest or rabbi. Some religious figures may exercise a great deal of political or social power, but committees of socially important believers in each community are generally responsible for the management of the mosque and its land. The real ecclesiastical power lies with the four orthodox schools of legal thought because they define the boundaries of Islamic law, theology, and belief.
Maliki: founded by Abd Allah Malik ibn Anas (ca. 715-95)
Hanifite: founded by An Numan ibn Thabit Abu Hanifa (ca. 700-67)
Shafi'ite: founded by Muhammad ibn Idris ash Shafii (767-820)
Hanbali: founded by Ahmad ibn Muhammad ibn Hanbal (780-855)
The Maliki school is centered around Medina and Malik's law book is the earliest known Muslim legal text. The Hanifi school is located in Iraq and stresses the use of individual opinion in rendering legal judgments. Shafii was a member of Muhammad's Quraysh tribe and was a distant relative of his. Shafii studied under Malik in Medina, but ended up following his own path, creating rules of analogy for the purpose of reaching legal opinions on matters which were not covered in direct statements made by Muhammad. Hanbal's legal school is centered in Baghdad and became prominent in Saudi Arabia because it is the only school accepted by the Wahhabi Muslims. It places the primary emphasis on the Hadith as the source of law and rejects later innovations made by other schools, scholars, and religious figures.
Religious Authority: Inherited & Mystical
Unlike the Sunnis, Shia Muslims have from the start regarded inherited, mystical elements as fundamental to the nature of religious authority. The term Shia is a shortened form of Shiat Ali, which means "the party of Ali." At the time of Ali's death in 661, that is probably all it was: a party or tendency of people who supported Ali's claims to the caliphate. Ali was Muhammad's first cousin, in some ways Muhammad's adoptive brother, the husband of his favorite daughter (Fatima), and father of his favorite grandsons. Moreover, Ali was regarded as more authentically representative of what Muhammad stood for and fought for, especially in contrast to the wealthy and worldly Umayyads.
After Ali died, his role was believed to have passed to his two sons, Hasan and Husain, who were also Muhammad's grandsons. Despite this, they did not take over the caliphate - that position went to Mu'awiya, who founded the Umayyad dynasty. After this time, the descendants of Ali became a principle focus of dissent and opposition to the Umayyads. Many came to believe that the Umayyads and following Islamic rulers were corrupt and had fallen away from the path set by Muhammad. Those who believed that justice and good government would only replace tyranny and corruption when the rightful heirs of Muhammad took control came to be known as the Shiites.
Why Religious Authority Matters
Differences in religious authority create significant differences in how a religion works. Rationalized, legalistic religions require certain types of critical arguments while charismatic, mystical religions require different critical arguments. Legalistic systems are vulnerable at their textual foundations; charismatic religions are vulnerable at their mystical claims. If you want to critique Islam, then, you need to know which Islam you are critiquing and where it is most vulnerable.-->