Modern research has proved that the origin of Sufism cannot be traced back to a single definite cause, and has thereby discredited the sweeping generalisations which represent it, for instance, as a reaction of the Aryan mind against a conquering Semitic religion, and as the product, essentially, of Indian or Persian thought. Statements of this kind, even when they are partially true, ignore the principle that in order to establish an historical connexion between A and B, it is not enough to bring forward evidence of their likeness to one another, without showing at the same time (1) that the actual relation of B to A was such as to render the assumed filiation possible, and (2) that the possible hypothesis fits in with all the ascertained and relevant facts. Now, the theories which I have mentioned do not satisfy these conditions. If Sufism was nothing but a revolt of the Aryan spirit, how are we to explain the undoubted fact that some of the leading pioneers of Mohammedan mysticism were natives of Syria and Egypt, and Arabs by race? Similarly, the advocates of a Buddhistic or Vedantic origin forget that the main current of Indian influence upon Islamic civilisation belongs to a later epoch, whereas Moslem theology, philosophy, and science put forth their first luxuriant shoots on a soil that was saturated with Hellenistic culture. The truth is that Sufism is a complex thing, and therefore no simple answer can be given to the question how it originated. We shall have gone far, however, towards answering that question when we have distinguished the various movements and forces which moulded Sufism, and determined what direction it should take in the early stages of its growth.

Let us first consider the most important external, i.e. non-Islamic, influences.

from:
THE MYSTICS OF ISLAM
by Reynold A. Nicholson
Routledge, Kegan Paul, London