How theological sects emerged in islam? What is the main cause for emergence of islamic sects?
The word madhhab literally means “the way to go, the place to go, the view, the doctrine, and the trend”. In Islamic terminology, it refers to the schools of Islamic law and creed, which are accepted as a self-consistent system of thought. Its plural form is “madhahib”. Mujtahid imams, known as the founders of the madhhab, are not the founders or messengers of a religion, but scholars who interpret the textual sources by using the Qur’an, Sunnah, ijmāʿ, qiyās, and other methods of inference and try to apply the limited number of texts to unlimited events of life. In general, madhhabs are named after their founders such as Maturidi and Ash’ari madhhabs in Islamic creed, and Ḥanafi, Shafiʿi, Maliki, and Ḥanbali madhhabs in Islamic law. The term “firqa (plural firāq)”, which means group, is also used to refer to the various sectarian groups that differ about the Islamic creed.
Iʿtiqād means belief. Believing in something, acknowledging someone, or a piece of news, and following it, is called “ʿaqīdah.” Its plural is ʿaqāid. In this case, belief and faith are synonymous. Iman as a term means to accept the religion of Allah by heart, that is, to confirm the things that the Messenger of Allah (saw) conveyed with his heart. In order to be treated as a Muslim in society, it is necessary to reflect in life on this belief existing in the heart.
ʿAqāid refers to the entirety of the Islamic rules and decrees related to faith. Acts of worship and deeds are outside the parameters of this scholarly discipline. The belief system of Islam is gathered in the sentence known as “amantu”. This consists of belief in the existence and oneness of Allah, His angels, His books, His prophets, the Hereafter, destiny, and that good and evil come from Allah.
The discipline of ʿaqāid, like all other Islamic sciences, was not written and codified in the age of bliss, which is the first period of Islam when the Messenger of Allah (saw) was alive. Since the revelation had not yet been ended, people who had problems in belief, worship, or human relations could go to the Messenger of Allah (saw), and the issue could be resolved in the light of revelation. The Companions were in complete submission to the Qur’an and the Messenger of Allah (saw) in matters of creed, as they did in all other matters. They would unreservedly accept a principle of belief brought by the Messenger of Allah. For instance, it is well known that Abu Bakr (ra) replied to the Meccan polytheists, who sarcastically asked about the Prophet’s miracle of ascension to the heavens with the following words, “If he said that he ascended to the heavens, then it is true. I believe in this and everything he brought from Allah.” On the other hand, it is also known that the Messenger of Allah (saw) forbade some Companions to discuss the issue of destiny for it may not be possible for the human mind to grasp some issues related to faith. Therefore, it is necessary to believe in such issues as they have been reported by the Prophet (saw).
After the demise of the Prophet, and thus the end of revelation, most of the Companions preserved their pure and clear Islamic beliefs. However, they began to face some challenges due to the increase in the population who had never met the Prophet (saw), the entry of new cultural circles into Islam, and the emergence of new social and political issues. The method through which the caliphs were selected, the incident of Hakam during the caliphate of Ali (ra),  and whether the person who commits a major sin (murtakib al-kabira) converts out of Islam or not, can be mentioned among such issues.
The first generations, which included the Companions and the followers, believed in the verses and hadiths related to the creed without subjecting them to interpretation. These early generations are called “Salafiyya.” They interpreted the verse “(Allah) Most Gracious is firmly established on the Throne” as Allah has a throne, but we do not know its nature. In like manner, they understood the verse “…the Hand of Allah is over their hands…” as Allah has a hand, but we cannot know its nature. The scholars of Islamic theology who lived in later periods, on the other hand, stated that such verses have metaphorical meanings; For instance, the Throne of Allah means His domination over the realm of beings, and that He has absolute power. By Allah’s hand, it is meant His power and Might. In this regard, they relied on Qur’anic verses stating, “…there is nothing whatever like unto Him…”
 See Şerafettin Gölcük, İslam Akaidi, Konya, 1989, p. 8 ff.
 Ta Ha, 20: 5; See al-Aʿraf, 7: 54; al-Tawba, 9: 129; Yunus, 10: 3; al-Rad, 13: 2; al-Furqān, 25: 59.
 Al-Fatḥ, 48: 10; See Āl ʿImrān, 3: 73; al-Mā’ida, 5: 64; al-Ḥadīd, 57: 29.
 Al-Shūrā, 42: 11; al-Ikhlas, 112: 4
Source: Basic Islamic Principles (ʿilmi ḥāl) According to the Four Sunni Schools With Evidence From The Sources of Islamic Law, Prof. Hamdi Döndüren, Erkam Publications