Surah 8: al-Anfāl
8:2 The faithful are only those whose hearts quake when God is remembered.
The faithful are those who fear God. In this verse He makes fear a precondition of faith, just as He says elsewhere: “Fear Me if you have faith” [3:175]. Fear is the protection of faith, the fortress of the religion, and the interceder for sins. When a heart does not have fear, that heart is in ruins, a source of trouble and deprived of God's gaze. In this verse He says that the faithful are those whose hearts fear and tremble at the remembrance of God.
In another place He says, “Those who have faith and whose hearts are serene in the remembrance of God” [13:28]. This alludes to the fact that the faithful are those whose hearts are at ease and rest in the remembrance of God. The former is the mark of the beginners, and the latter is the description of the advanced. At the beginning of his traveling, the servant always weeps, wails, and moans. He weeps so much in dread of separation that the call “Fear not!” [41:30] reaches his secret core. He comes away from the dread of separation to the repose of union. In this station he is at ease and delighted, his heart at rest. This is why He says, “whose hearts are serene in the remembrance of God.”
It has also been said that whose hearts quake is the description of the desirer, and whose hearts are serene is the attribute of the desired. Whose hearts quake is the watchword of the folk of the Shariah, and whose hearts are serene is the blanket of the folk of the Haqiqah. Whose hearts quake is the station of the travelers, and whose hearts are serene is the mark of the snatched away. The travelers are in the road of the Shariah in the station of service hoping for blessings, and the snatched away are caressed on the carpet of the Haqiqah, given access to proximity and nearness by the Patron of Blessings.