Who Are the Deobandis?
Question

Are the Deobandis part of Ahl al-Sunnah? Are they within the folds of Islam?
Praise be to Allah.Who are the Deobandis?
The Deobandis are one of the groups of Muslims. This group is connected to and named after the University of Deoband – Dar al-Ulum – in India. It is an intellectual school of thought that is deeply rooted, and everyone who graduated from that university was influenced by its academic characteristics, so that they became known as Deobandis. 
Foundation of the University of Deoband
The University of Deoband was founded by a group of Indian scholars after the British had put a stop to the Islamic revolution in India in 1857 CE. Its establishment was a strong reaction against western advancement and its materialistic civilization in the Indian Subcontinent, aimed at saving the Muslims from the dangers of these circumstances, especially when Delhi, the capital, had been destroyed following the revolution and the British had taken full control of it. 
The scholars feared that their religion might be assimilated, so Shaykh Imdadullah al-Muhajir al-Makki and his student Shaykh Muhammad Qasim al-Nanatuwi, and their companions, drew up a plan to protect Islam and its teachings. They thought that the solution was to establish religious schools and Islamic centers, thus al-Madrasah al-Islamiyyah al-Arabiyyah was established in Deoband as a center for Islam and Shari`ah in India at the time of British rule. 
Prominent figures of the Deobandi intellectual school
The most prominent figures of this intellectual school:

Muhammad Qasim
Rashid Ahmad al-Kankuhi
Husayn Ahmad al-Madani
Muhammad Anwar Shah al-Kashmiri
Abu’l-Hasan al-Nadvi
Al-Muhaddith Habib al-Rahmaan al-A’zami

Beliefs of the Deobandis 
With regard to basic tenets of belief (‘aqidah) of the Deobandis, they follow the madhhab of Abu Mansur al-Maturidi.
They follow the madhhab of Imam Abu Hanifah with regard to fiqh and minor issues. 
They follow the Sufi tariqahs of the Naqshbandiyyah, Chishtiyyah, Qadiriyyah and Saharwardiyyah with regard to spiritual development. 
Thoughts of the Deobandis
The thoughts and principles of the Deobandi school may be summed up as follows: 

Preserving the teachings of Islam and its strength and rituals.
Spreading Islam and resisting destructive schools of thought and missionary activity.
Spreading Islamic culture and resisting the invading British culture.
Paying attention to spreading the Arabic language because it is the means of benefiting from the sources of Islamic shari’ah.
Combining reason and emotion, and knowledge and spirituality. (See al-Mawsu’ah al-Muyassarah fil Adyan wal Madhahib, 1/308) 

What is al-Maturidiyyah?
Because the Deobandis follow the Maturidi madhhab with regard to belief (‘aqidah), we have to define what al-Maturidiyyah is:
This is a philosophical (kalami) group which is named after Abu Mansur al-Maturidi. It is based on using rational and philosophical proof and evidence in disputes with opponents from among the Mu’tazilah, Jahamiyyah and others to establish the truths of religion and Islamic ‘aqidah (belief). 
Sources of Islam according to al- Maturidiyyah
With regard to sources, the Maturidiyyah divide the bases of religion into two categories depending on the source: 

Divine or rational: these are matters which are established independently by reason and the reports follow that. This includes issues of Tawhid and the Divine attributes. 
Legislative matters or transmitted reports, These are matters which reason states may or may not exist, but there is no way to prove rationally that they exist, such as prophethood, the torment of the grave and issues of the Hereafter. It should be noted that some of them regarded prophethood as coming under the heading of rational issues. 

It is obvious that this is contradictory to the methodology of Ahl al-Sunnah wa’l-Jama’ah , because the Quran, Sunnah and consensus of the Companions are the sources of guidance in their view. This is in addition to their bid’ah (innovation) of dividing the sources of religion into rational matters vs. transmitted reports, which was based on the false notion of the philosophers who assumed that the religious texts contradict reason, so they tried to mediate between reason and the transmitted reports. This led them to force reason into fields where it has no place, so they came up with false rulings which contradicted shari’ah, and that led them to say that they did not know what the texts mean and that only Allah knows their meaning, or to misinterpret them altogether. In the view of Ahl al-Sunnah wa’l-Jama’ah, on the other hand, there is no contradiction between sound reason and the sound transmitted reports. (See al-Mawsu’ah al-Muyassarah fi’l-Adyan wa’l-Madhahib al-Mu’asirah, 1/99) 
Attitude of Ahl al-Sunnah towards the Maturidiyyah
It was narrated from the Prophet (peace and blessings of Allah be upon him) that this ummah would split into seventy-three sects, all of which would be in the Fire apart from one. The Prophet (peace and blessings of Allah be upon him) explained that the saved group is the Jama’ah, which is the group that follows the same path as the Messenger (peace and blessings of Allah be upon him) and his Companions. 
Undoubtedly Ahl al-Sunnah wa’l-Jama’ah, who adhere to the Quran and Sunnah in terms of both knowledge and actions, are the saved group , and this description applies to them, i.e., they adhere to that which the Messenger (peace and blessings of Allah be upon him) and his Companions adhered to in terms of knowledge and actions. 
It is not sufficient for an individual or group merely to claim to belong to the Sunnah whilst going against the methodology of the salaf, namely the Sahabah and Tabi’in. Rather it is essential to adhere to their methodology in knowledge, action, approach and spiritual development. 
The Maturidiyyah are one of the groups whose opinions include true and false views, and some things that go against the Sunnah. It is known that these groups vary with regard to the truth, how near or far they are; the closer they are to the Sunnah, the closer they are to the truth and the right way. Among them are some who went against the Sunnah with regard to basic principles, and some who went against the Sunnah with regard to more subtle issues. There are some who refuted other groups who are farther away from the Sunnah, so they are to be praised with regard to their refutation of falsehood and what they have said of truth, but they have overstepped the mark in so far as they have rejected part of the truth and gone along with some falsehood. So they have refuted a serious bid’ah by means of a lesser bid’ah, and refuted falsehood with a lesser form of falsehood. This is the case with most of the philosophers (ahl al-kalam) who claim to belong to Ahl al-Sunnah wa’l-Jama’ah…” (From the words of Shaykh al-Islam Ibn Taymiyah, al-Fatawa, 1/348) 
What is our duty towards the Maturidiyyah and the Deobandis?
There remains one important question to be answered, which is: what is our duty towards the Maturidiyyah and groups who hold similar beliefs such as the Deobandis and others?
The answer varies according to differences in the persons involved. 
If someone is stubborn and propagates his bid’ah, then we must warn others about him and explain where he has gone wrong and deviated. But if he does not propagate his bid’ah and it is clear from his words and actions that he is seeking the truth and striving for that purpose, then we should advise him and explain to him what is wrong with this belief, and guide him in a manner that is better; perhaps Allah will bring him back to the truth. This advice is included in the words of the Prophet (peace and blessings of Allah be upon him): “Religion is sincerity (or sincere advice).” We [the Sahabah] asked, “To whom?” He said, “To Allah and His Book, and His Messenger, and to the leaders of the Muslims and their common folk.” (Narrated by Muslim, 55)
And Allah knows best.