Who was al-Hallaaj?
Question

                Who was Mansoor Hallaj . What is his role in Islamic History?
Praise be to Allah.Al-Hallaaj was al-Husayn ibn Mansoor al-Hallaaj, who was known as Abu Mugheeth, or Abu ‘Abd-Allaah. He grew up in Waasit, or it was said in Tastar, and he was connected with a group of Sufis including Sahl al-Tastari, al-Junayd, Abu’l-Hasan al-Noori and others. 
He traveled to many places, including Makkah and Khurasaan, and India where he learned sihr (magic, witchcraft). He finally settled in Baghdaad, where he was killed. 
He learned magic in India, and he was a trickster and cheat. He deceived many ignorant people thereby, and they were attracted by him, until they thought that he was one of the greatest awliya’ (close friends or “saints”) of Allaah. 
He is liked by most of the Orientalists, and they think that he was killed wrongfully because, as we shall see below, his beliefs were close to Christian beliefs and he preached a similar message. 
He was executed in Baghdad in 309 AH, because it was proven by his own confession and otherwise that he was a kaafir and a heretic. 
The scholars of his time were agreed that he was to be executed because of the words of kufr and heresy that were narrated from him. 
The following are some of the things that he said: 
1-He claimed to be a prophet, then he went further and said that he was God. He used to say, “I am Allaah,” and he commanded his daughter-in-law to prostrate to him. She said, “Should I prostrate to someone other than Allaah?” He said, “There is a god in the heavens and a god on earth.”
2-He believed in incarnation and union with the Divine, i.e., that Allaah was incarnated in him, and that he and Allaah had become one and the same – exalted be Allaah far above that.
This is what made him acceptable to the Christian Orientalists, because he shared their belief in incarnation, for they believe that God was incarnated in Jesus (‘Eesa, peace be upon him). Hence al-Hallaaj spoke of divine nature and human nature as the Christians do. 
Some of the lines of verse that he composed said: 
“Glory be to the one Whose human form manifested the secret of His divinity
Then He emerged among His creation in the form of one who eats and drinks.” 
When Ibn Haneef heard these lines, he said, “May the curse of Allaah be upon the one who said this.” It was said to him, “This is the poetry of al-Hallaaj.” He said, “If this is what he believes, then he is a kaafir.” 
3-He heard someone reciting a verse from the Qur’aan, and he said, “I am able to compose something like that.”
4-Another of his lines of poetry says:
“People formed different beliefs about God, and I believe in everything that they believed.” 
These words imply that he approved of and believed in all forms of kufr that the misguided sects of humanity believe in, but it is a contradictory notion that no sound mind can accept. How can anyone believe in Tawheed and shirk at the same time? 
5-He said things which denied the pillars and basic principles of Islam, namely prayer, zakaah, fasting and Hajj.
6-He used to say that the souls of the Prophets had been reincarnated in the bodies of his companions and students. So he would say to one of them, “You are Nooh”; and to another, “You are Moosa”; and to another, “You are Muhammad.”
7-When he was taken out to be executed, he said to his companions, “Do not worry about this, for I will return to you after thirty days,” He was executed and he never came back.
Because of these sayings and others, the scholars were agreed that he was a kaafir and a heretic, and for this reason he was executed in Baghdad in 309 AH. Similarly, most of the Sufis denounced him and denied that he was one of them. Among those who denounced him was al-Junayd, and he was not mentioned by Abu’l-Qaasim al-Qushayri in his book in which he mentioned many of the Sufi shaykhs. 
The one who strove to have him executed and who held a council in which he ruled that he deserved to be executed was al-Qaadi Abu ‘Umar Muhammad ibn Yoosuf al-Maaliki (may Allaah have mercy on him). Ibn Katheer praised him for that and said, “One of his greatest and most correct judgements was his ruling that al-Husayn ibn Mansoor al-Hallaaj was to be executed.” (al-Bidaayah wa’l-Nihaayah, 11/172) 
Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him) said: “Whoever believes what al-Hallaaj believed in and agrees with the ideas for which al-Hallaaj was executed, is a kaafir and an apostate, according to the consensus of the Muslims. For the Muslims executed him because of his belief in incarnation, union with the Divine and other heretical beliefs, such as his saying, ‘I am Allaah,’ and, ‘There is a god in the heavens and a god on earth.’ … Al-Hallaaj performed extraordinary feats and various kinds of magic, and there are books of magic which are attributed to him. In conclusion, there is no dispute among the ummah that whoever believes that Allaah can be incarnated in a human being and be as one with him, or that a human being can be a god, is a kaafir whose blood it is permissible to shed. On this basis al-Hallaaj was executed.”
(Majmoo’ al-Fataawa, 2/480) 
He also said: “We do not know of anyone among the imams of the Muslims who spoke well of al-Hallaaj, neither among the scholars nor among the shaykhs. But some of the people did not comment on him because they did not know about him.”
(Majmoo’ al-Fataawa, 2/483) 
For more information, see Taareekh Baghdaad by al-Khateeb al-Baghdaadi, 8/112-141; al-Muntazam by Ibn al-Jawzi, 13/201-206; Siyar A’laam al-Nubalaa’ by al-Shihaabi, 14/313-354; al-Bidaayah wa’l-Nihaayah by Ibn Katheer, 11/132-144 
And Allaah is the Guide to the Straight Path.