the sins committed in the past

"The believing people should always be fearful of the sins committed in the past and should weep for them since we do not know which of our sins can bring about out destruction. The Imam (A) has not informed us of any sin that we can ignore and remain bereft of the Mercy of Allah (S.w.T.). However, it is certain that the sin for which we have not repented would be sure to prevent our salvation. So we must enter the domain of Allah (S.w.T.)’s grace through the door of repentance. We should sincerely repent for those sins, which are forgotten by us and especially seek forgiveness of those sins that we remember."

SOURCE: GREATER SINS V1 | AYATOLLAH DASTAGHAIB SHIRAZI (R)