Past sins are forgiven
There are many Hadith which describe the excellence of Shab Qadr. Those who spend this night in worship and remembrance of Almighty Allah, their past sins are forgiven, provided that person is a believer and is sincere. Faith and sincerity are basic conditions for the forgiveness of past sins. Thus, there is a Hadith in Sahih Bukhari:
Translation: It is narrated on the authority of Hadhrat Abu Hurairah (May Allah be well pleased with him), he says: The Holy Prophet (Sallallahu alaihi wa sallam) said: The person who spends Shab Qadr in worship (of Allah Most High) in a state of faith with the intention of reward, all the past sins of that person are forgiven. (Sahih Bukhari, Hadith No. 35)