In The Name of Allah, The Most Merciful, The Bestower of Mercy.

Allah, 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]

Meaning: Through calamities, diseases and the divine laws; but they neither turn in repentance nor (abandon) the evil they are upon, nor do they learn a lesson regarding what will benefit, so that they may act upon it; nor regarding what is harmful to them, so that they may abandon it. Allah, The Most High, put them to the test- and this is Allah’s way with all the nations- through poverty, poor health, calamities, and through commands and prohibitions, so that they might turn to Allah, but they neither repent nor learn a lesson.

In this verse is proof that Iman increases and decreases, and that it is obligated to a believer to examine his Iman- pay close attention to it, improve and (act upon the legislated Shariah means to) increase it, so that it rises constantly. 

A slightly paraphrased excerpt from “Tayseer Al-kareem Ar-Rahmaan Fee Tafseer kalaam Al-Mannaan by Imam As-Sadi, may Allah have mercy upon him.