Allaah says (Interpretation of the meaning):
“But when a decisive Soorah (explaining and ordering things) is sent down, and fighting (Jihaad — holy fighting in Allaah’s Cause) is mentioned (i.e. ordained) therein, you will see those in whose hearts is a disease (of hypocrisy) looking at you with a look of one fainting to death”
At times of ease and plenty, the Muslims may be joined by those who seek to make worldly gains,
and they do not want to make the word of Allaah prevail over the word of kufr. These people may conceal their real nature from many of the Muslims,
and the most effective means of exposing them is jihaad,
because jihaad means sacrificing oneself, but these hypocrites are only indulging in hypocrisy in order to save themselves.
Exposing the hypocrites was one of the major purposes that Allaah wanted the believers to achieve on the day of Uhud.
Allaah says (interpretation of the meaning):
“Allaah will not leave the believers in the state in which you are now, until He distinguishes the wicked from the good”
[Aal ‘Imraan 3:179]
Ibn al-Qayyim said:
“This means: Allaah will not leave you in the state in which you are, where the believers are indistinguishable from the hypocrites, until the believers are made distinct from the hypocrites, as they were distinguished by the test on the day of Uhud; and Allaah will not disclose to you the unseen matters by which the one group is distinguished from the other, for they are distinguished from one another in the knowledge of the unseen that He has, but He wants to distinguish them from one another in a clear and visible manner, so that His unseen knowledge will become known and visible.”