First:

The greatest good deed is faith in Allah and His Messenger, and the greatest evil deed is disbelief. Faith is a positive reality: a person is not outwardly a believer until he manifests the foundation of faith—the testimony that there is no deity worthy of worship except Allah, and the testimony that Muhammad is the Messenger of Allah. Nor is he inwardly a believer until he affirms this in his heart, so that doubt is absent both outwardly and inwardly, alongside the performance of righteous deeds. Otherwise, he is like those of whom Allah said:

“The Bedouins say, ‘We believe.’ Say, ‘You have not believed; rather, say, “We have submitted,” for faith has not yet entered your hearts.’”

And like those of whom the Most High said:

“Among the people are those who say, ‘We believe in Allah and the Last Day,’ but they are not believers.”

And like those of whom He said:

“When the hypocrites come to you”—and the rest of the verse.

Disbelief is the absence of faith, by the agreement of the Muslims, whether a person believes and voices its opposite, or neither believes anything nor says anything. In this respect, there is no difference between the position of Ahl al-Sunnah wa-l-Jamāʿah, who hold that faith consists of inward and outward words and deeds, and the position of those who hold that it consists solely of the heart’s belief, such as the Jahmiyyah and most Ashʿarīs; or solely of verbal profession, such as the Karrāmiyyah; or of both together, such as the jurists of the Murjiʾah and some Ashʿarīs. For these, together with the scholars of hadith, the majority of Mālikī, Shāfiʿī and Hanbalī jurists, the general body of Sufis, and groups of theologians—both theologians of the Sunnah and theologians outside the Sunnah among the Muʿtazilah and the Khawārij,