(Al-Fath'h) 48 : 2
لِيَغْفِرَ لَكَ اللَّهُ مَا تَقَدَّمَ مِنْ ذَنْبِكَ وَمَا تَأَخَّرَ وَيُتِمَّ نِعْمَتَهُ عَلَيْكَ وَيَهْدِيَكَ صِرَاطًا مُسْتَقِيمًا
For Allah to forgive you your offenses, what has passed before and what is in the future, and to complete His Mercy upon you, and to guide you to the Straightpath.
The Mercy mentioned in the verse is Adhikr. Only with Adhikr one can become believer, and seek forgiveness for the offences happened before by remembering them and presenting them with soul before Impartial Lord; and keep the rest of life without happening offences. See explanation 36: 10-11; and 42: 52.