Allaah (The Most High) said:
أَوَلَا يَرَوۡنَ أَنَّهُمۡ يُفۡتَنُونَ فِى ڪُلِّ عَامٍ۬ مَّرَّةً أَوۡ مَرَّتَيۡنِ ثُمَّ لَا يَتُوبُونَ وَلَا هُمۡ يَذَّڪَّرُونَ
See they not that they are tried once or twice every year? Yet, they turn not in repentance, nor do they learn a lesson (from it).’’ [9:126]
(Put to trial) by way of calamities and diseases and (by way of the) the divine laws. However, they neither turn in repentance nor (abandon) the evil they are upon, nor do they learn a lesson with regards to what would benefit them, so that they may act upon it; nor (do they learn a lesson with regards to) what is harmful to them, so that they may abandon it.
Allaah (The Most High) put them to the test-and this is Allaah’s way with all the nations-through poverty (or loss in wealth); loss of health; calamities; through commands and prohibitions, so that they might turn to Allaah; but they neither repent nor learn a lesson.
In this Ayah is proof that Imaan increases and decreases and that it is obligated upon the believer to examine his Imaan- paying close attention to it, improving and making it grow (through the means that Allaah has prescribed for its improvement and growth), so that it constantly rises.
[Paraphrased and abridged. For further details, see Tayseer Al-kareem Ar-Rahmaan Fee Tafseer kalaam Al-Mannaan of Imaam Sadi (rahimahullaah)]