The State of the People of Faith and Their Reward Allah states:

إِنَّمَا يُؤْمِنُ بِـَايَـتِنَا

(Only those believe in Our Ayat,) means, who accept them as true,

Before approaching verse 16: تَـتَجَافٰى جُنُوْبُهُمْ عَنِ الْمَضَاجِعِ يَدْعُوْنَ رَبَّهُمْ خَوْفًا وَّطَمَعًا (Their sides remain apart from their beds. They call their Lord with fear and hope - 16), it should be borne in mind that disbelievers, Mushriks and the deniers of the day of Qiyamah were warned in the verses previous to it. After that, starting from: اِنَّمَا يُؤْمِنُ بِاٰيٰتِنَا ! (Only those people believe in Our verses - 15) mentioned there are special attributes of sincere believers and the high ranks reserved for them. Pointed to in the cited verse is one such attribute of these believers: They leave the comfort of their beds, rise and get busy with the remembrance of Allah in submission and supplication - because, they are apprehensive of His displeasure and punishment while remaining hopeful of His mercy and reward. This very combination of fear and hope keeps them returning to Dhikr and Du'a' repeatedly, anxiously and animated at the same time.

The Salah of Tahajjud