The Ash'ari theology (Arabic الأشاعرة al-asha`irah) is a school of early Muslim speculative theology founded by the theologian Abu al-Hasan al-Ash'ari (d. 324 AH / 936 AD). The disciples of the school are known as Ash'arites, and the school is also referred to as Ash'arite school.
It was instrumental in drastically changing the direction of Islamic theology, separating its development radically from that of theology in the Christian world.
Factors affecting the spread of the school of thought include a drastic shift in historical initiative, foreshadowing the later loss of Muslim Spain and Columbus' landing in the Western Hemisphere - both in 1492. But the decisive influence was most likely that of the new Ottoman Empire, which found the Asharite views politically useful, and were to a degree taking the advantages of Islamic technologies, sciences, and openness for granted. Which, for some centuries after as the Ottomans pushed forth into Europe, they were able to do - losing those advantages gradually up until The Enlightenment when European innovation finally surpassed and eventually overwhelmed that of the Muslims.
While al-Ash'ari was opposed to the views of the Mu'tazili school for its over-emphasis on reason, he was also opposed to the views of certain orthodox schools such as the Zahiri (literalist), Mujassimite (anthropomorphist) and Muhaddithin (traditionalist) schools for their over-emphasis on taqlid (imitation) in his Istihsan al‑Khaud:
His book The Revival of the Religious Sciences in Islam was the cornerstone of the school's thinking, and combined theology, skepticism, mysticism, Islam and other conceptions, discussed in depth in the article on Islamic philosophy.
Other works of universal history from al-Tabari, al-Masudi, Ibn al-Athir, and Ibn Khaldun himself, were quite influential in what we now call archaeology and ethnology. They worked in a relatively modern style that historians of the present would recognize.
The influence of the Asharites is still hotly debated today.
Most agree that the Asharites put an end to philosophy as such in the Muslim world, but permitted these methods to continue to be applied to science and technology. The 12th-to-14th century marked the peak of innovation in Muslim civilization. During this period many remarkable achievements of engineering and social organization were made, and the ulema began to generate a fiqh based on taqlid ("imitation based on authority") rather than on the old ijtihad. Eventually, however, modern historians think that lack of improvements in basic processes and confusion with theology and law degraded methods. The rigorous means by which the Asharites had reached their conclusions were largely forgotten by Muslims before The Renaissance, due in large part to the success of their effort to subordinate inquiry to a prior ethics - and assume ignorance was the norm for humankind.
Modern commentators blame or laud Asharites for curtailing much of the Islamic world's innovation in sciences and technology, then (12th century to 14th century) leading the world. This innovation was not in general revived in the West until The Renaissance, and emergence of scientific method - which was based on traditional Islamic methods of ijtihad and isnad (backing or scientific citation). The Asharites did not reject these, amongst the ulema or learned, but they stifled these in the mosque and discouraged their application by the lay public.
The Asharites may have succeeded in laying the groundwork for a stable empire, and for subordinating philosophy as a process to fixed notions of ethics derived directly from Islam - perhaps this even improved the quality of life of average citizens. But it seems the historical impact was to yield the initiative of Western civilization to Christians in Europe.
Some argue, however, that the Asharites not only did not reject scientific methods, but indeed promoted them. Ziauddin Sardar points out that some of the greatest Muslim scientists, such as Ibn al-Haytham and Abū Rayhān al-Bīrūnī, who were pioneers of the scientific method, were themselves followers of the orthodox Ash'ari school of Islamic theology.