by Allamah Sayyid Muhammad Husayn Tabatabai
Among the companions of the Prophet, Ali is known particularly for his eloquent exposition of gnostic truths and the stages of the spiritual life. His words in this domain comprise an inexhaustible treasury of wisdom. Among the works of the other companions which have survived there is not a great deal of material that concerns this type of question. Among the associates of Ali, such as Salman Farsi, Uways Qarani, Kumayl ibn Ziyad, Roshaid Hajari, Maytham Tammar, Rabi'ibn Khaytham. However, there are figures who have been considered by the majority of the Sufis, Sunni and Shi'ite alike, as the heads of their spiritual chain (silsilah) after Ali.
After this group there appeared others, such as Tawus Yamani, Shayban Ra'i, Malik ibn Dinar, Ibrahim Adham, and Shaqiq Balkhi, who were considered by the people to be saints and men of God. These men, without publicly talking about gnosis and Sufism, appeared externally as ascetics and did not hide the fact that they had been initiated by the earlier group and had undergone spiritual training under them.
After them there appeared at the end of the 2nd/8th century and the beginning of the 3rd/9th century men such as Bayazid Bastami, Ma'ruf Karkhi, Junayd Baghdadi and others like them, who followed the Sufi path and openly declared their connection with Sufism and gnosis. They divulged certain esoteric sayings based on spiritual vision which, because of their repellent external form, brought upon them the condemnation of some of the jurists and theologians. Some of them were imprisoned, flogged, and even occasionally killed. Even so, this group persisted and continued its activities despite its opponents. In this manner gnosis and the "Way" (Tariqah, or Sufism) continued to grow until in the 7th/13th and 8th/14th centuries it reached the height of its expansion and power. Since then, sometimes stronger and at other times less so, it has continued its existence to this very day within the Islamic world.
Gnosis or Sufism as we observe it today first appeared in the Sunni world and later among the Shi'ites. The first men who openly declared themselves to be Sufis and gnostics, and were recognized as spiritual masters of Sufi orders, apparently followed Sunnism in the branches (furu') of Islamic law. Many of the masters who followed them and who expanded the Sufi orders were also Sunnis in their following of the law.
Even so, these masters traced their spiritual chain, which in the spiritual life is like the genealogical chain of a person, through their previous masters to Ali. Also the results of their visions and intuitions as transmitted to us convey mostly truths concerning divine unity and the stations of the spiritual life which are found in the sayings of Ali and other Shi'ite Imams. This can be seen provided we are not affected by some of the striking and even sometimes shocking expressions used by these Sufi masters and consider the total content of their teachings with deliberation and patience. Sanctity resulting from initiation into the spiritual path, which Sufis consider as the perfection of man, is a state which according to Shi'ite belief is possessed in its fullness by the Imam and through the radiance of his being can be attained by his true followers. And the Spiritual Pole (qutb), whose existence at all times is considered necessary by all the Sufis - as well as the attributes associated with him - correlates with the Shi'ite conception of the Imam. According to the saying of the Household of the Prophet, the Imam is, to use the Sufi expression, Universal Man, the manifestation of the Divine Names and the spiritual guide of the lives and actions of men. Therefore, one could say, considering the Shi'ite concept of walayat, that Sufi masters are "Shi'ite" from the point of view of the spiritual life and in connection with the source of walayat although, from the point of view of the external form of religion they follow the Sunni schools of law.
This is an excellent book on Islamic Spirituality or "The Inner Journey" of Sufism that will give you glimpses of the light of Allah from various prominent authors on the subject:
It is necessary to mention that even in classical Sunni treatises it has sometimes been said that the spiritual method of the "Path," or the "techniques" whereby one comes to know and realize himself, cannot be explained through the external forms and teachings of the Shari'ah. Rather these sources claim that individual Muslims themselves have discovered many of these methods and practices, which then have become accepted by God, such as is the case with monasticism in Christianity. Therefore each master has devised certain actions and practices which he has deemed necessary in the spiritual method, such as the particular type of ceremony of being accepted by the master the details of the way in which the invocation is given to the new adept along with a robe, and the use of music, chanting and other methods of inducing ecstasy during the invocation of the Divine Name. In some cases the practices of the Tariqah have outwardly become separated from those of the Shari'ah and it may seem difficult for an outsider to see the intimate and inward relation between them. But by taking into consideration the theoretical principles of Shi'ism and then studying in depth the basic sources of Islam, namely the Quran and the Sunnah, he will soon realize that it is impossible to say that this spiritual guidance has not been provided by Islam itself or that Islam has remained negligent in clarifying the nature of the spiritual program to be followed.