The systematization of Sunni theology in the tenth century was done in reaction to the emergence of heterodox schismatic groups in previous centuries, particularly Mu'tazilah. The founder of Ash'ariyyah, Abu al-Hasan (873-935), was formerly a Mu'tazilite. He wrote a number of important books which became the foundation of Ash'arite theology such as the Kitab al-Ibanah (The Book of Elucidations) and also an extensive work on the views of various Islamic schools and sects called Maqalat al-Islamiyyin (Doctrines of the Muslims).
Another major figure in the development of Ash'arite theology was the Sufi theologian and jurist al-Ghazzali (1058-1111). Through al-Ghazzali and other prominent theologians - such as Al-Baqillani (d.1013), al-Baghdadi (d.1038), al-Djuwayni (d.1085) and al-Shahrastani (d.1153) - Ashariyyah spread throughout the Sunni Islamic world. It is now dominant in Iraq, Syria, Egypt and Northwest Africa, and has a strong presence in Central Asia and Anatolia and to a lesser extent in India and Pakistan.
Ash'ariyyah theology represents a reaction against the extreme rationalism of the Mu'tazilah. It holds that human reason should fall under the authority of divine revelation. Human reason is incapable of discerning good and evil; the goodness or evil of a particular action depends upon God's declaring it to be so. Humanity can only acquire religious truths through revelation.
A second aspect of Ash'ariyyah theology concerned the nature of the divine attributes. Contrary to the Mu'tazilites, who understood Qur'anic references to God's physical attributes metaphorically, Ash'ari theology argued for the veracity of these attributes while rejecting all crudely anthropomorphic conceptions of God.
Thirdly, contrary to Mu'tazilah theology, Ash'ariyyah taught that the Qur'an was eternal and, therefore, uncreated. Human actions, however, are entirely dependent upon God's providing the means and power to act. This teaching had the purpose of preserving the doctrine of divine omnipotence, but gradually led to the formation of a deterministic outlook.
Along with the Maturidiyyah School of theology, Ashariyyah remains the dominant source of theology in the Sunni world. The majority of those who follow the Malikite jurisprudential madhhab, which is comprised of 13% of worldwide Sunni Muslims, and some 75% of those who follow the Shafi'ite jurisprudential madhhab, which constitutes some 33% of worldwide Sunni Muslims, and a very small proportion of those who follow the Hanafite and Hanbalite jurisprudential madhhabs follow the Ash'arite school of theology.