Yet Jahm and those who agreed with him claimed that such a person is inwardly a believer … (1) and that mere knowledge and affirmation in the heart constitute faith that warrants reward on the Day of Resurrection, without any spoken profession or outward action. This is false according to both revelation and reason, as has been explained at length elsewhere. Scholars of the early generations, such as Wakīʿ, Ahmad, and others, declared those who held this view to be disbelievers. The Prophet, may Allah bless him and grant him peace, said:

“Indeed, in the body is a piece of flesh: if it is sound, the whole body is sound; and if it is corrupt, the whole body is corrupt. Indeed, it is the heart.”

He thus explained that soundness of the heart necessarily entails soundness of the body. If the body is not sound, this indicates that the heart is not sound. Since a believing heart is sound, it follows that someone who professes faith but does not act upon it does not have a believing heart. Even someone under coercion, when it comes to expressing faith, must speak of it to himself and privately with someone he trusts. It must also show in his facial expressions and unguarded words, as ʿUthmān said. But if no trace of it ever appears in either his words or his deeds, this indicates that there is no faith in his heart.

This is because the body follows the heart. Nothing becomes firmly established in the heart without its necessary effects and implications appearing on the body in some way, even if an impediment prevents all its effects from appearing. The cause that brings about those effects remains present, whereas the impediment does not accompany a person as inseparably as his heart does. Its manifestation is impossible only in certain circumstances, when