Rumi: True and False Sufis
False Sufis steal the words of the Sufis and present them as their own, and thus trick people with the beautiful teachings, which they are merely imitators of. They do not have true knowledge of that which they are speaking. Many of the great Sufi Shuyukh speak of deceptive false Sufis - these people take the title of "Sufi" because it gives them a certain amount of prestige and power, and they take advantage of those who cannot tell false Sufism from the real thing.
On the topic of false Sufis, Mawlana Jalaluddin Rumi says:
A disciple who is trained by a man of God will have a pure and purified spirit. But he who is trained by an imposter and hypocrite and who learns theory from him will be just like him: despicable, weak, incapable, morose, without any exit from uncertainties, and deficient in all his senses. "As for the unbelievers -- their protectors are idols, that bring them forth from the light into the shadows." (Qur'an 2:257).
[From the "Fihi ma fihi," translated by W. C. Chittick in "The Sufi Path of Love: the Spiritual Teachings of Jalaluddin Rumi," p. 145]
Furthermore, on the topic of false Sufis and the harm they do, Jalaluddin Rumi says (in poetry):
You are the disciple and guest of someone who in his vileness will steal away all your attainments.
He is not victorious -- how will he make you victorious? He will not give you light, he will make you dark.
Since he has no light, how can others receive light through associating with him?
Like a blind man who cures eyes: With what will he anoint your eyes other than wool? [...]
He has no scent or trace of God, but his claims are greater than those of Seth or Adam.
The devil himself is embarrassed to appear before him; he keeps on saying, "We are of the saints and even greater."
He steals many of the words of the dervishes, so that people may think he really is someone.
In his talks he even cavils at Bayazid; Yazid himself is ashamed of him. (*)
He is destitute of the bread and provisions of heaven: God has not thrown him a single bone.
[From the Mathnawi of Jalaluddin Rumi, Book I, vv. 2265-68, 72-76, translated by W. C. Chittick in "The Sufi Path of Love: the Spiritual Teachings of Jalaluddin Rumi," p. 145-6.]
(*) Bayazid refers to the great Wali Allah, Bayazid al-Bistami; Yazid refers to the oppressive ruler.
Therefore we see that a great Shaykh such as Jalaluddin Rumi warns us of the false Sufis. False Sufis may even be more prevalent today than they were in the past. We must distinguish between true Sufis, who follow the Shari`ah, and false Sufis, who often do not.