His father Shaykh Isma'il was a renowned alim of his time and he took special care to give the right type of education to his brilliant son in Qur'an, Hadith, fiqh, usul, ad-din, etc. from his early boyhood. The father took special care to educate the boy on the lines recommended by the Shari'a and he asked him not to get in touch with the Mu'tazilite group of scholars who were very active in those days and who were spreading their controversial ideas.
But Abu'l-Hasan frustrated all attempts of his father, went to Baghdad and became a disciple of the principal advocate of the Mu'tazilite group, Abu'l-Wahhab al-Jubba'i. Under his guidance he learnt philosophy, logic, and literature as well, and soon made his name as a good speaker and philosopher. But the Mu'tazilities had already, by that time, lost the power and patronage that they had previously enjoyed, and they were being hunted out by the enraged theologians and other Muslim scholars, who bore an inborn hatred towards the sect.
The Mu'tazilities used to interpret the Qur'an and Hadith in a purely rationalistic way. They were known as 'free thinkers' and they were influenced by the old Greek and other ancient philosophers from whom they borrowed freely when propounding their theories. in the name of the development of Islamic philosophy and Arabic literature, they borrowed so freely and indiscriminately from these alien sources that they lost touch with the Qur'an and Sunnah. They started interpreting the Qur'an in a new way and held that the Qur'an was a 'created' thing and that the Miraj took place in dream or by the imagination. In short, these Mu'tazilites by their resort to 'free thinking' corrupted the roots of Islam from within and they convinced the luxury-loving Abbasid Khalifs, particularly Khalif al-Ma'mun, to accept their idea of the interpretation of the Qur'an.
Since he was earlier trained by the Mu'tazilites, he knew their weaknesses and sensitive issues and thus he started replying to their points one after another. He introduced the science of al-Kalam that is scholastic theology which saved the Muslim nation from the cursed rationalism of the Mu'tazilites.
Soon he became widely acclaimed as a scholar throughout the Muslim world. The 'Ulama' all accepted him as their leader in their fight against the Mu'tazilites.
Abu'l-Hasan al-Ash'ari was a born genius. He wrote more than one hundred books on different subjects of Islam. Of these, his Maqalat al-Islamiyya is the greatest book on 'ilm al-Kalam. In this book, he compared Greek philosophy with Islamic philosophy and pointed out those things which are irrelevant to the requirements of Islam and made 'ilm al-Kalam the basis of our philosophy. In this respect, he is the father of 'ilm al-Kalam or Islamic theology, and Imam al-Ghazzali is its perfector.
The ideas and teachings of Abu'l-Hasan al-Ash'ari made their impact in the East. In its initial stage the Ash'ari school of thought was strongly opposed by the Mu'tazilites, but gradually, with the strong, support it received from the khalifate, particularly from men like Nizamu'l-Mulk and Imam al-Ghazzali, his doctrine became very popular all over the Muslim world. Al-Ash'ari and al-Ghazzali were criticised for their strong opposition to the study of science and philosophy for the sake of study only, without considering the need to study religion and the Shari'a. But it is they who saved the Muslim nation from the onslaught of the Mu'tazilites, who were taking the nation to secularism.