The ascetic practices began to be supplemented with mystical ideas from the 8th to the 9th Centuries. Centres of mysticism developed in Kufa, Basra, Khurasan, Baghdad, Syria, Lebanon and Egypt under the influence of location and individual ideologists.
Al-Hasan al-Basri (d. 728) was possibly the first ideologist of mystical Sufism. His ideas revolved around great and necessary fear of Allah. In that sense he was only beginning to depart from the first strand that I mentioned above.
Ibrahim Ibn Adham (d. 777) lived in Khurasan in eastern Iran and later Syria. He abandoned a former life of luxury in Khurasan to journey to Syria practising asceticism. Shaqiq of Balkh (d. 801), also of eastern Iran, was possibly the first to develop the notion of certain attitudes denoting particular mystical ‘states’ that later 'caught on' in Sufi ideology. The first attitude he articulated was one of “trust in God” or “tawakkul”.
Rabi’a of Basra’s (d. 801) focus was on love of God. She was the first noted female Sufi and still inspires today with her concise poetry and aphorisms concerning that love as she experienced it.
Al-Harith Ibn Asad al-Muhasibi (d. 857) moved from Basra to Baghdad to found the Baghdad School whose focus tended to be on spiritual accountability and self examination and understanding, hence his nickname related to examination or investigation (al-Muhasibi).
Al-Ghazali (d. 1111), the extremely multi-talented 11th and 12th Century religious and secular scholar, later recognised al-Muhasibi’s contribution in what he called the “science of hearts”. There was room in Sufism, he considered, for all of (scientific) introspection, theology and Qur’anic and Hadith study. He further articulated attitudes (such as repentance, pious fear of God, love, resolution and fear with hope) that would be held to produce particular mystical ‘states’ and ‘stations’ in later Sufism.
Al-Junayd’s (d. 910) focus was on knowledge of God. He was initially a Baghdadi follower of Muhasibi who developed a school of ‘sober’ Sufis (as opposed to the "intoxicated" Sufis (see below)). He continued to follow the idea of Sufism as an intellectual and sober quest for understanding of God. This he saw as a process of returning to God and "life" from a state of absence of God and "death".
Abu Yazid al-Bistami (d. c. 875) led the "intoxicated" Sufis who believed that the mystic was actually able to become one with God (rather than merely subsist in Him/Her/It as soberly proposed by the other school). Al-Hallaj (d. 922) took this to such an extreme that he was executed at the indirect instigation of Islamic jurists, who saw him as mad and a blasphemer (which he probably was). He proposed that the mystic could achieve this oneness without erecting all the ‘pillars’ of Islam (he (or she) could perform a ‘spiritual’ hajj, for instance, in his (or her) own home without having to outwardly perform a hajj). For a time, the teachings of this "intoxicated" school led to a decline in the prestige of Sufism. Al-Hallaj was essentially executed for saying that he was God (so he essentially suffered the fate of Jesus).
Finally, after over a century, the many-talented al-Ghazali managed to reinstate Sufism as legitimate in Islam. He was reacting to excessive formalism and intellectualism in Islamic rules. He sought to actually experience God’s presence and finally settled on Sufism after rejecting theology, philosophy and Esoteric Isma’ili teaching methods (in all of which he apparently excelled). He was able to integrate his practice into a scheme permitted by Islamic theology and laws. His form of Sufism was directed towards a fullness of worship of God and fellowship with others. He fully articulated the way of Sufism including its stations and states also discussed by al-Qushayri (d. 1074) and others in the 11th Century.
The main stations were held to be repentance, abstinence, renunciation, wariness or pious awe of God, meekness (tawadu’), humility (khushu’), sincerity, constancy and courtesy. Others included earnest spiritual striving, solitude and withdrawal, silence, hope, sorrow and fear. The last "station", which may also be the first of the "states" is satisfaction or acceptance. Later states include servanthood to God, desire for God or seeking of God and finally love and spiritual yearning.
Major Sufi poets after this rebirth include Ibn ‘Arabi (d. 1240), the Egyptian, Omar Ibn al-Farid (d. 1235), the Iranian (and writing in Persian), Farid al-Din Attar (d. 1220), and from Anatolia the famous Jalal al-Din Rumi (d. 1273) (also writing in Persian).
After the early asceticism was thus modified and as a more popularist form of Sufism thus developed and until the late 19th Century, most Muslims in the Middle East were affiliated with at least one of the many Sufi "Schools" (usually as "lay" members).