Because Peter was genuinely repentant. There was clear evidence of God’s forgiveness when on the day of Pentecost, Peter was privileged to give a bold witness about Jesus.
(‏ لوقا ٢٢ :‏ ٦٢‏ )‏ وَكَانَ هُنَالِكَ دَلِيلٌ وَاضِحٌ عَلَى غُفْرَانِ ٱللهِ هذَا فِي يَوْمِ ٱلْخَمْسِينَ حِينَ نَالَ بُطْرُسُ ٱمْتِيَازَ تَقْدِيمِ شَهَادَةٍ جَرِيئَةٍ عَنْ يَسُوعَ .‏