Later it was expounded on by various Islamic scholars, such as Ibn Taymiiya in the 13th century, but it did not become institutionalized until the mid-18th century when the theories promulgated by the radical cleric Muhammad ibn Abd al-Wahhab were accepted and imposed as the state religion of his realm by the founder of the House of Saud.
Another, in the 18th century, was Muhammad ibn Abd al-Wahhab, who advocated purging Islam of modern accretions and relying strictly on the Koran and hadith (the record of the prophet's words and deeds).