Sufism is the major expression of mysticism in Islam. While Sufism developed out of the fusion of Qur’anic ascetic tendencies and the vast fund of Christian (and other) mystical sayings present throughout the classical world, by approximately the 10th century it had become a uniquely Islamic feature. Major writers such as al-Ghazali and Ibn al-ʿArabi took this heritage and molded it both into a normative tradition for Islam as a whole (by wedding it to the Prophet Muhammad’s life experience) and, in the case of Ibn al-ʿArabi, into completely new spiritual paths. These interpretations of mysticism were critical in the vast conversion to Islam that happened during the period 1000–1800. Although other factors were involved as well, including trading by Muslims and the Islamic educational system, this conversion happened largely at the hands of the Sufis, especially holy men and healers, and thus the Muslim world is still largely Sufi or Sufi-influenced. Starting in the 19th century, however, and culminating in the mid-20th century, large numbers of Muslims abandoned Sufism, accusing it of being fundamentally anti-Islamic and even polytheistic. Today although Sufis still constitute the bulk of world Muslims, and they are visible throughout the non-Muslim world as well, their belief system is under attack as never before.