Shaykh Muḥammad b. Ṣāliḥ al-ʿUthaymīn, may Allāh have mercy upon him, was asked: “Who are the Muʿtazilah?”

He replied:

“The Muʿtazilah are an innovated group, they speak about Allāh, the Speech of Allāh, and regarding the actions of Allāh [in a manner] which opposes the way of Ahl al-Sunnah wa al-Jamāʿah. Their heads are ʿAmr b. ʿUbayd (d. 144) and Wāṣil b. ʿAṭāʾ (d. 131). They were called the Muʿtazilah because they withdrew and isolated themselves from al-Ḥasan al-Baṣrī (d. 110) who affirmed that the perpetrator of a major sin has not left īmān, rather he is a believer who is deficient in faith, or he is a believer with regards to his faith but a sinner with respect to his perpetration of a major sin. So, they withdrew from this gathering; the gathering of al-Ḥasan al-Baṣrī, and they pronounced the infamous opinion: the perpetrator of a major sin is in a state between two states; he is not a believer neither is he a disbeliever, but despite that he is eternally in the Fire.

So, they converge with the Khawārij in stating that the perpetrator of a major sin is in the Fire forever, but the Khawārij declare that he is a disbeliever who is outside the fold of Islām, while they [the Muʿtazilah] declare that he is outside the fold of Islām but they do not state that he is a disbeliever. Instead, they say that he is in a state between two states. So, they affirm this state which opposes the Speech of Allāh the Almighty and Exalted in which He stated, the Blessed and Most High: {He is the One Who created you all, then some of you are disbelievers and some of you are believers} [al-Taghābun (64): 2] – there is no third category in which one is neither a disbeliever nor a believer except according to the view of these Muʿtazilah who innovated in the religion of Allāh and His Divine Law that which is not from it.”

Source: Fatāwá Nūr ʿalá al-Darb (1: 687, no. 574).