Who are the Marturidis?
The beliefs of Islam are quite clear, as expounded in the Quráan and Sunnah, and as understood by the Sahaabah (Radhiyallahu unhum).
It is this set of simple beliefs that are the fundamentals of Imaan. Whoever adheres to these straightforward Aqaaid is considered a Muslim and one who rejects any of the essential beliefs of the Sahaabah (Radhiyallahu unhum) is out of Islam.
It is this set of simple beliefs that is the criterion of whether a person is Muslim or not.
As Islam spread far and wide, many of the works of Greek philosophy were translated into Arabic. Muslims were for the first time exposed to the rational theology of the Greeks.
This created a climate where arguments for and against Islamic beliefs were examined in the light of Greek rational thought.
This gave birth to a new discipline called Ilmul Kalam.
Thus Ilmul Kalam was not a purely Islamic discipline, having not existed during the time of the Sahaabah (Radhiyallahu unhum).
It was a branch of knowledge that arose out of the need of the time, i.e. The need to refute Greek theology on the basis of rational principles.
In the process of examining Islamic beliefs on the basis of Greek philosophy, many new questions arose.
These delicate and intricate questions were never posed before, nor were they discussed during the age of the Sahaabah (Radhiyallahu unhum). There were no explicit texts of the Quráan and Sunnah in reply to these intricate questions.
Discussions around these intricate issues led to the formation of many groups.
In this field of knowledge the Ahlus Sunnah Wal Jama'ah were represented by two Ulama, Imaam Abul Hasan Ashari and Imaam Abu Mansoor AlMarturidi (Rahimahumallah).
They successfully covered these delicate issues in their discourses, and clarified the standpoint of the Ahlus Sunnah Wal Jama'ah in these matters.
Thus, as far as Ilmul Kalam is concerned, the outlook of the Ahlus Sunnah Wal Jama'ah is confined to the works of these two Imaams of Ilmul Kalam.
Allamah Haythami states in his Fatawa Hadithiyyah ".whoever is upon a path other than the path of Ahl al-Sunna wa l-Jama'a, Ahl al-Sunna wa l-Jama'a meaning the followers of Sheikh Abul Hasan Ash'ari and Abu Mansur Maturidi, the two Imams of Ahl al-Sunna" (Haytami, al-Fatawa al-hadithiyya, 280).
The followers of Imaam Abu Mansoor Marturidi are called Marturidi. The Ulama of Deoband are Marturidi.
While these two Imaams differed on minor issues, it was merely a difference of interpretation, for the subject matter of Ilmul Kalam is rational thought, which itself allows for differences of interpretation.
Taken with thanks from http://www.askimam.org/fatwa/fatwa.php?askid=09fab3cf050c61884b281d0e458fedfc