Allah Ta’ala says regarding the quality of a believer in Surah Zumr (39),
“Is one who worships devoutly during the hour of the night prostrating himself or standing, who takes heed of the Hereafter, and who places his hope in the Mercy of his Lord (like the one who does not)? Say: “Are those who know equal to those who do not know? Only the people of intellect take head.” (39:9)
Allah Ta’ala has classed the people of knowledge as the one who stands in vigil at night, praying towards Allah with utmost humility and servitude. What is evident in such an ayah is that those who are void of these qualities of praying at night, humility and servitude are essentially devoid of the title of Mu’min.
Allah Ta’ala himself mentions who these mu’min are in Surah Anfaal,
“Al-Mu’minoon are only those who, when mention of Allah is made (to them) their hearts start tremble (out of fear)” (8:2)
Under this passage of Surah Anfaal, Allah Ta’ala shows specific qualities of a Mu’min. Imam Razi rahimahullah has enumerated them as:
a. Their heart trembles out of fear at the mention of Allah.
b. When the Ayaat are recited to them, their Imaan increases.
c. They have complete reliance on Allah Ta’ala.
d. They establish prayer.
e. They spend from that which Allah Ta’ala has given them. (Mafateeh al Ghayb 15/450)
Similarly Allah Ta’ala has explained the qualities of Mu’minoon in many different places.
“Only those believe in Our Signs, who, when they are reminded thereof, fall down in prostration, and glorify praises of their Lord, nor are they puffed up with pride. Their limbs give up their beds, while they call unto their Lord fearing and hoping, and they spend from that which he have sustained them with. (32:15-16)
These verses are using the term “Innama” which is a term of restriction. In terminology it is called “Kalimah al-Hasr” (a restricting word). Its purpose is to restrict a condition which would otherwise remain general. So when the verse says that “only those believe who…”, the restriction helps us understand that the Mahsoor (that which is restricted) is such a significant condition that absence of it would render someone devoid of the title of “believer”.
Hence we understand from these verses that dhikr of Allah, standing in the night for vigil, establishing prayer etc. are such important qualities that one who does not take significance of them, should not be afforded the title of a true believer. In essence, this is also an encouragement for us to bring these qualities in our lives and get rid of all that disobedience from our heart so that we can attain the certification of being a “Mu’min” from none other than Allah Ta’ala himself.