"For, Believers are those who, when Allah is mentioned, feel a tremor in their hearts, and when they hear His Signs rehearsed, find their faith strengthened, and put (all) their trust in their Lord."
Wajl of the Heart
Aishah, may Allah be pleased with her has said, "Wajl can be likened to the spark of a burning palm-leaf. When one of you feels that, let him resort to supplications." (Ibn Jarir, Alusi, Shawkani, Manar)
Imam Razi writes: "Of 'fear' there are two kinds: fear of punishment and fear of Allah's Greatness and Majesticity. The first is that of the sinner. As for the second kind, there is no escape from it for anyone, be it a Prophet or an angel close to Allah. It is the first kind that is alluded to here. Suddi has said that he is a believer who, when he intends a sin or wrong, is reminded of Allah and told, 'Fear Allah,' then his heart is shaken by fear and he desists." (Qurtubi, Ibn Kathir)
Umar bin Abdul Aziz wrote to someone: "Faith is (the other name of) superogatories, obligatories and the Law. Whoever observed them fully, completed his faith. Whoever didn't, didn't complete his faith." (Zamakhshari)
Razi adds: "The strength and quantity of faith depends upon the evidences that one has, of which of course, there is no end. It is a shore less ocean. Every time one discovers the wisdom behind one creation, he moves on to the wisdom behind the creation of another, and so on, each step causing increase in his faith."
Someone asked Hasan al-Busri, "O Abu Said, are you a believer?" He replied, "There are two kinds of faiths. If you are asking me about belief in Allah, His angels, His Books, His Messengers and in Paradise, Fire, resurrection and reckoning, then, yes I am a believer. But if you are asking me about this verse then I am not too sure if I belong to that category." (Zamakhshari) Therefore, adds Qurtubi, the true believer is one who will be judged (in the Hereafter) as one deserving Paradise. Until then his state is a secret unto all but Allah.
However, when belief and unbelief are mentioned as a basis of membership of the Muslim community or in connection with legal rights and responsibilities as necessary corollaries of that membership, a clear line of demarcation is drawn on the basic minimum of belief regardless of the quality of belief.
Compiled From: "Tafsir Ishraq Al-Ma'ani" - Syed Iqbal Zaheer, pp. 162-164