Why are Muslims of the Mu'tazilite school not considered to be among the people of Sunnah, even though they are monotheists and followers of the Messenger, may Allah's prayers and peace be upon him, and their differences may lie in the interpretation of verses, which is an avenue for ijtihad?

The Mu'tazilah were an Islamic sect that emerged during the Umayyad era and flourished during the Abbasid era. They relied on reason and prioritized rational understanding over transmitted texts in comprehending creed. Their principles are five: Tawhid (negating attributes), Justice (denying divine decree), al-Manzilah bayna al-Manzilatayn (a grave sinner is neither a believer nor an unbeliever), Enforcing the Threat (grave sinners of the Muslim community will abide eternally in Hell), and Enjoining Good and Forbidding Evil (permitting rebellion against rulers).

They impugned the Companions who participated in the Battle of the Camel and claimed that God is obligated to do what is best for His servants. Islamic scholars refuted their views. Scholars differed on whether to declare them disbelievers, with some holding that they are not. Shaykh al-Islam Ibn Taymiyyah held that the root of their misguidance was turning away from what the Messenger brought, and that negating attributes and denying the Beatific Vision and the Qur'an constitute disbelief. The school of thought is rife with errors and violations of Islamic law, and there is no room for ijtihad (independent reasoning) in the presence of definitive texts.