The word Fitnah means a trial or test. A Fitnah itself is not something evil. However, the results of a Fitnah could be evil or good. This word could also mean to follow a deviated opinion. Thus, a Fitnah is a test which may take on various forms. It is also used for something which overwhelms people with falsehood. So, when used in this way- in the verse- it means ‘this causes disturbances among people.’

(  إِلَّا أَن قَالُوا وَاللَّهِ رَبِّنَا مَا كُنَّا مُشْرِكِينَ… ) (…except they will say, “By Allah, our Lord, we were not those who associated.”): The results of the test were such that they said on the Day of Judgment: “Verily, we were never polytheists.” They lie, and will try, up till these moments, to swear “By Allah”. They are in fact trying to free themselves, like Allah (swt) says:

 إِذْ تَبَرَّأَ الَّذِينَ اتُّبِعُواْ مِنَ الَّذِينَ اتَّبَعُواْ وَرَأَوُاْ الْعَذَابَ وَتَقَطَّعَتْ بِهِمُ الأَسْبَابُ

(Consider the Day) when those who misled others will disown their followers—when they face the torment—and the bonds that united them will be cut off) (Al-Baqara: 166)