Probably the best-known establishment figure in classical Sufism was al-Ghazali (c. 1058–1111), who was a Sunni theologian at a prestigious Baghdad college, the Nizamiyya, and who, after going through a spiritual crisis that caused him to leave it all for fourteen years, converted to Sufism and became an ardent exponent of it in later life. As al-Ghazali was uniquely qualified to marry the mainstream Sharia-abiding element of Sufism with its radical antinomian wing, his influence on Sufism is immense. Al-Ghazali’s methodology, especially in his great work Ihya `ulum al-din (The revival of religious knowledge), was to take major categories of life (eating, drinking, death, etc.) and to spiritualize their primary manifestations through anecdotes and creative reinterpretations of preexisting materials. From his time until the present the vast bulk of Muslims have been Sufis, and it is hard to point to a major intellectual or religious figure in Islam that was not a Sufi or at least influenced by Sufism until the middle of the 19th century. Al-Ghazali in many ways made Sufism intellectually and socially respectable.

It was during this period (11th–12th centuries) that the process of Sufis converting people to Islam began to be a major factor. Although the ascetic literature gives examples of conversion to Islam from the early period, following al-Ghazali larger numbers of Sufis began to organize themselves into brotherhoods (tariqa). These organizations have continued to be critical in the spread of Islam to the present time, and they are most usefully compared to orders (in the Roman Catholic Church) or to denominations (in the Protestant world). Groups of mendicant Sufis (called faqirs, meaning “poor” or dervishes) began to gather at the borders of the Muslim world, in Central Asia, in West and East Africa, in India, and eventually in Southeast Asia. In each one of these regions the process of conversion was remarkably similar. Sufi holy men (following in the footsteps of Christian monks of the early period) gained the reputation of constituting a spiritual elite. They had healing and intercessory powers, and they could—and did—confront Islamic rulers and their religious intellectuals (the ʿulamā). This confrontation was often to the benefit of the Sufis, as they could position themselves as local heroes and gain converts.

It was not at all unusual during this period and to the present day for these Sufi holy men to cultivate an entire aura of sanctity around their persons. This sanctity was usually conveyed by the term baraka, or “blessing.” Sometimes this baraka accrued to a given person, who would then pass it down to his descendants, or to a place, usually his tomb or a place closely associated with his teachings or miracles during his lifetime and thereafter. Baraka was something that continued to be in effect and constituted a strong draw to the people of the region and sometimes far beyond (as pilgrims). People would come for miracles, healing, and, most especially, to gain the intercession of the holy man for their sins against the moment on the Day of Judgment when they would be judged. It was commonly hoped that the holy man’s general baraka would protect his followers or those who had revered his holy site. On a popular level, all sorts of amulets, prayers, and paraphernalia would be sold and cultivated at such sites.

At the same time as these popular trends were growing, developments took place in Sufi cosmology and metaphysics. The Spanish Muslim figure Ibn al-ʿArabi (1165–1240) was arguably the most intellectually rigorous individual that Islam has ever produced, and in his voluminous works he solidified abstract notions of God’s being, God’s relation to the cosmos, and its interrelation with humanity. Bringing together many strands of Sufism and writing in the most careful yet often obscure manner (because his many opponents believed him to be destructive to Islam), Ibn al-ʿArabi formulated a theory that later became known as wahdat al-wujud(although he himself did not use that term). Wahdat al-wujud, as Ibn al-ʿArabi formulated it, emphasized the absolute existence of God as opposed to the finite and fleeting existence of all created creatures, and God’s absolute oneness. This latter emphasis has opened the doctrine of wahdat al-wujud to charges that it is pantheistic. Ibn al-ʿArabi’s emphasis and enthrallment with the nature of God’s existence can possibly lead one in that direction. However, it is unlikely that the man himself was a pantheist.

Wahdat al-wujud constituted only part of a larger intellectual framework that Ibn al-ʿArabi developed. As William Chittick, one of Ibn al-ʿArabi’s most comprehensive interpreters writing in English, has stated: “Ibn al-ʿArabi’s teachings come together on the issue of human perfection, which is none other than for human beings to be fully human … the degree to which they achieve this goal establishes their worth as God’s servants and vice-regents and determines their situation in this world and the next.” In this manner, Ibn al-ʿArabi developed the doctrine of al-insan al-kamil, the Perfect Man, which became the goal to which Sufis aspired. Although the archetypal Perfect Man was of course Muhammad, Ibn al-ʿArabi allowed for the possibility of others gradually attaining types of perfection. Moving away from mystical annihilation of the earlier periods, he proposed that the gnostic Sufi, through his cognizance of God, could (hypothetically) attain a level of perfection that would be similar to that of the Prophet Muhammad. This doctrine continued to have ramifications throughout Sufism during the following centuries.

Almost at the same time as Ibn al-ʿArabi was formulating his theories, the major Sufi poet and thinker Jalal al-Din Rumi (1207–1273) was composing his epic ecstatic verses on the theme of universal love. Rumi, who was a refugee from the onslaught of the Mongol invasions, based himself in Anatolia (in present-day Turkey), where he dictated to his disciples Mathnawi (in Persian, Masnavi), in which the formulation of ecstatic Sufism took final form. Rumi avoids any Hallaj-like proclamations of complete union with God, but he emphasizes a type of Islamic universalism that has served as spiritually attractive for many even to the present day. His followers, the Mevlevis, developed the emblematic pattern of dance (the “whirling dervishes”) in his honor, and they helped to bring music and dance into the repertoire of mainstream Sufism. However, it should be noted that these elements, so attractive to outsiders, have been problematic within the context of Islam.

A typical story of Rumi is told that he saw a group of priests dressed in black, and a number of his followers made negative comments about their appearance. Rumi, on the other hand, stated: “There are no people in the world who are more generous.” Even though they would eventually be sentenced to hellfire, he affirmed that “nonetheless, when the sun of divine favor suddenly shines on them, they will immediately become illuminated and have white (bright) faces. [poem] ‘If a hundred year old infidel beholds you, he prostrates himself and quickly becomes a Muslim.’” Indeed, the group of priests did convert to Islam, and Rumi’s reaction was: “God Most High conceals blackness in whiteness and He gives whiteness a place within blackness.”

Rumi’s importance within Sufism also represents the growth of Sufi poetry and epic, which, although it had its roots in early Arabic ascetic poetry, was closely linked to the revival of the Persian language in the 11th through 14th centuries. Poets such as Saʿdi (c. 1213–1291 or 1292), Nizami Ganjavi (1141–1209), and Hafez (d. 1390) developed this tradition and contributed greatly to the literary tradition of Sufism. Many writers used Sufi themes or heroes in their stories or allegories, and, in this manner, Sufism and Sufi ideas came to dominate the discourse of Islam, especially in lands influenced by Persian (Iran, Central Asia, India, Turkey). Sufism thus moved away from its early roots as a low-class phenomenon and became the language of the courtly elite while the Sufi lifestyle became the ideal to which they aspired.