Allaah (The Most High) said:

يَوْمَ تُبْلَى السَّرَائِرُ

The Day when all the secrets (deeds, prayers, fasting, etc.) will be examined (as to their truth). [86:9]

Imaam Sadi (rahimahullaah) said: That is: The secrets of the hearts will be examined. The good and evil that used to be in the hearts will be manifested on the faces (of the people).  Allaah (The Most High) said:

يَوْمَ تَبْيَضُّ وُجُوهٌ وَتَسْوَدُّ وُجُوهٌ

On the Day (i.e. the Day of Resurrection) when some faces will become white and some faces will become black.’’ [3:106]

In this worldly life, many affairs are concealed and are not manifested to the people, but on the day of judgement, the righteousness of the righteous (ones) and the immorality (or wickedness) of the wicked (ones) will be manifested and the affairs will be made public. [Source: Tayseer Al-Kareem Ar-Rahmaan Fee Tafseer Kalaam Al-Mannaan]

Also see Shaikh Uthaymeen’s (rahimahullaah) Tafseer of Ayah 9 Surah At-Taariq and what he stated about the importance of rectification of the hearts and the corruption of the khawaarij.