(Taken from the book Evolution of Fiqh p144 by Bilal Philips)
Ahmad Ibn Taymiyah (1263-1328CE) was the foremost among the reformers of this period. Because of his challenge of the status quo, many of his contemporaries declared him an apostate and had the authorities jail him repeatedly.
Ibn Taymiyah was, however, one of the greatest scholars of his time. Initially, he had studied fiqh according to the Hanbali madh-hab, but did not restrict himself to it.
He studied the sources of Islamic law in depth and mastered all the Islamic sciences which were known at that time. Furthermore, he examined the writings of various sects which had broken off from Islam, studied the religious books of the Christians, the Jews and their various sects and wrote extensive critiques on all of them.
Ibn Taymiyah also took part in the Jihad against the Mongols who had occupied the eastern and northern provinces of the Abbasid state and were at the time threatening Egypt and North Africa.
Ibn Taymiyah’s students were among the greatest Islamic scholars of their time and carried on to the next generation the banner of Ijtihad and a return to the pure sources of Islam which he had raised. Among them was Ibn Qayyim, a great scholar in the fields of Fiqh and hadith, adh-Ddhahabi, a master in the science of hadith criticism and Ibn Katheer, a master in Tafseer, history and hadith.