(Al-Fath'h) 48 : 2
لِيَغْفِرَ لَكَ اللَّهُ مَا تَقَدَّمَ مِنْ ذَنْبِكَ وَمَا تَأَخَّرَ وَيُتِمَّ نِعْمَتَهُ عَلَيْكَ وَيَهْدِيَكَ صِرَاطًا مُسْتَقِيمًا
For forgiving Allah to you from your offences what is passed before and what is in future, and to complete His Mercy upon you, and to guide you to the Straight path.
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.