Reverting to Islam is always a big step because it involves a person breaking through his or her past beliefs and seeing the light of Islam. Accordingly, Allah rewards such people by forgiving all their past sins and starting their accounts anew. Consider the following ahadith of the prophet (s):
Islam demolishes all the previous evil deeds and so do migration (for Allah’s sake) and Hajj (pilgrimage to Makkah). (Sahih Muslim, Vol.1, Chapter 52, Page 18).
Narrated Abu Sa‘îd Al-Khudri Allah’s Messenger said, “If a person embraces (converts to) Islam sincerely, then Allah shall forgive all his past sins, and after that starts the settlement of accounts: the reward of his good deeds will be ten times to seven hundred times for each good deed, and an evil deed will be recorded as it is unless Allah forgives it.” (Sahih Al-Bukhari, Vol.1, Hadith No. 40A).
About the rewards, it applies also for those who are steadfast on their religion. Narrated Abu Hurairah Allah’s Messenger said: “If any one of you improves (follows strictly) his Islamic religion, then his good deeds will be rewarded ten times to seven hundred times for each good deed and a bad deed will be recorded as it is.” (Sahih Al-Bukhari, Vol.1, Hadith No. 40B).