Innama almu'minoona allatheena itha thukira Allahu wajilat quloobuhum wa-itha tuliyat AAalayhim ayatuhu (surah Anfal: 2)

Tafseer Tafheem-ul-Quran by Syed Abu-al-A'la Maududi

(8:2) The true believers are those who, when Allah's name is mentioned, their hearts quake, and when His verses are recited to them their faith grows, *2 and who put their trust in their Lord;

Innamal mu`minoonal lazeena izaa zukiral laahu wajilat quloobuhum wa izaa tuliyat `alaihim Aayaatuhoo zaadat hum eemaananw wa `alaa Rabbihim yatawakkaloon

*2). A man's faith grows as he is able to confirm and submit to the command of God which he comes across. This is especially so where he submits to commands which go against his own personal predilections. A man's faith attains great heights if instead of trying to twist and distort the commands of God and the Prophet (peace he on him), he develops the habit of accepting and submitting to all the commands of God and the Prophet (peace be on him); if he strives to shape his conduct to the teachings which go against his personal opinions and conceptions, which are contrary to his habits, interests and convenience, which are not in consonance with his loyalties and friendships. For if he hesitates to respond positively to God's command, his faith is diminished. One thus learns that faith is not a static, immobile object. Nor is every, act of belief, or unbelief, of the same quality. An act of belief may be better or worse than another act of belief. Likewise, an act of unbelief may differ in quality from another act of unbelief. For both belief and unbelief, are capable of growth and decline.
All this concerns the essence of belief and unbelief. However, when belief and unbelief are mentioned as a basis for membership of the Muslim community or in connection with legal rights and responsibilities as necessary corollaries of that membership, a clear line of demarcation has to be drawn between those who believe and those who do not. In this respect the determination of who is a believer and who is not will depend on the basic minimum of belief regardless of quality of belief. In an Islamic society all those who believe will be entitled to the same legal rights and will be required to fulfil the same duties regardless of the differences in the quality of their faith. Likewise, all unbelievers - regardless of the differences in the quality of their unbelief - will be placed in the category of unbelievers disregarding the question whether their unbelief is of an ordinary quality or an extremely serious one.