Imam As-Sadiq (a.s.) said:
"اَلْمُؤْمِنُ بَيْنَ مَخافَتَيْنِ: ذَنْبٌ قَدْ مَضَى لايَدْرِى ما صُنْعُ اللّهُ فيه و عُمْرٌ بَقِىَ لايَدرِى ما يَكْتَسِبُ فيه"
A faithful believer is always anxious concerning two things: for his past sins and how God will treat him for these sins; and for his remaining life and the uncertainty about how he will spend it.1
The most manifest sign of belief is feeling responsibility, both for what has transpired and for the duties and obligations that should be fulfilled.
Those who possess these two feelings will always think about compensation for past negligence, as well as finding the best possible way for using future opportunities. These thoughts are instrumental in the continuous development and progress of a man or a nation.
The ones who are oblivious of their past mistakes and have no intention or plans to improve the future lead a poor and miserable life.