This is discussed at length elsewhere.

When those who adopted some of their innovated speculative theology—such as al-Ash‘arī and those among the followers of Ahmad, Mālik, and al-Shāfi‘ī who followed his path—began following their approach in holding that rational inquiry is obligatory and that faith cannot be attained without it, Abū Ja‘far al-Simnānī, one of the leading Ash‘arī scholars, said:

“This position is a vestige of Mu‘tazilism that remains in the school among those who hold it. This is because al-Ash‘arī had been a Mu‘tazilī, a student of Abū ‘Alī al-Jubbā’ī. He then abandoned that position for the doctrine of Ibn Kullāb and others like him who affirmed the divine attributes and divine decree, held that those who commit major sins will not remain in Hell forever, and upheld similar fundamentals concerning which the Mu‘tazilah departed from the main body of Muslims.

The originators of innovated speculative theology—which contradicts the Quran and Sunnah and was condemned by the early generations and the imams—were the leading figures of the Jahmiyyah, the Mu‘tazilah, and others like them. The Mu‘tazilah are both Qadarīs and Jahmīs, whereas Jahm and his followers are Jahmīs who espouse compulsion in human actions. Al-Ash‘arī himself had belonged to them. When he parted company with them, exposed their shameful doctrines, demonstrated their contradictions, and followed the approaches of Abū Muhammad ibn Kullāb and others like him, he opposed them in the strongest terms on questions of divine decree, threats of punishment, and religious designations and rulings, just as the Jahmiyyah, Dirāriyyah, Najjāriyyah, and similar groups opposed them on these matters.