Shaykh Ṣāliḥ al-Fawzān (حفظه الله) said¹:

Fitnah² [in the Arabic language means] examination.

Thus, a person is not left to say: ‘I believe,’ ‘I have accepted Islām,’ ‘I am a Muslim’ and ‘I am a believer’; [rather,] it is inevitable that he is tried and examined.

 
If he endures upon his faith and is firm upon it when fitan [strikes], then certainly he is truthful in his faith. As for if he swerves when fitan [strikes] and deviates from his religion, then he is a hypocrite, and is untrue in his faith, as Allāh said:

وَمِنَ النَّاسِ مَن يَقُولُ آمَنَّا بِاللَّـهِ وَبِالْيَوْمِ الْآخِرِ وَمَا هُم بِمُؤْمِنِينَ ﴿٨﴾ يُخَادِعُونَ اللَّـهَ وَالَّذِينَ آمَنُوا وَمَا يَخْدَعُونَ إِلَّا أَنفُسَهُمْ وَمَا يَشْعُرُونَ ﴿٩﴾ فِي قُلُوبِهِم مَّرَضٌ فَزَادَهُمُ اللَّـهُ مَرَضًا ۖ وَلَهُمْ عَذَابٌ أَلِيمٌ بِمَا كَانُوا يَكْذِبُونَ ﴿١٠﴾

‘And of mankind, there are some [hypocrites] who say: ‘We believe in Allah and the Last Day’ while in fact they believe not. They [think to] deceive Allah and those who believe, while they only deceive themselves and perceive it not! In their hearts is a disease [of doubt and hypocrisy], and Allah has increased their disease. A painful torment is theirs because they used to tell lies.’
[al-Baqarah, 2:8-10]

Hence, this is the Sunnah of Allāh.

Allāh [also] said:

مَّا كَانَ اللَّـهُ لِيَذَرَ الْمُؤْمِنِينَ عَلَى مَا أَنتُمْ عَلَيْهِ حَتَّى يَمِيزَ الْخَبِيثَ مِنَ الطَّيِّبِ ۗ وَمَا كَانَ اللَّـهُ لِيُطْلِعَكُمْ عَلَى الْغَيْبِ ﴿١٧٩﴾

‘Allāh will not leave the believers in the state in which you are now, until He distinguishes the wicked from the good. Nor will Allah disclose to you the secrets of the ghayb [unseen].’
[Ālī ʿImrān, 3:179]

Therefore, Allāh imposes trials and tests upon the people, upon the believers and the Muslims in order to differentiate the truthful from the untruthful, the true believer from the untruthful hypocrite and goodness from evil. This is the wisdom of Allāh.

Therefore, fitan occurs amongst the people due to this divine reason, and if fitan did not occur, the truth would have been mistaken for falsehood, the believer [would have been mistaken] for the hypocrite, and this [i.e. the truth and/or the believer] would not have been differentiated from that [i.e. falsehood and/or the hypocrite].

Endnotes:

1. Sharḥ Ḥadith Inna Kunna fi-al-Jāhiliyyah, pg. 8-9.
2.  Fitan (فتن) is the plural of fitnah (فتنة).

Translated by: Abū Wāʾil Musa Shaleem