Mercy is not understood by most, even though we long to experience its wonder. The best explanation I have ever read was that of John Paul II in his Letter, Rich in Mercy.
It is so easy for us to think that mercy means that some wonderful person is kind to some miserable and terrible person. For example, in the parable of the prodigal son, some might think simply that the Father is wonderful and the son is terrible, so the wonderful Father overwhelms the son with a merciful kindness that makes the terrible and miserable son feel better about himself. In fact, this is not what mercy is. Mercy is when the wonderful Father (looking over the hills for his son to return) knows that down deep the son is wonderful, too. And, the Father shows to the son the respect for his down deep dignity, because that is actually the way the Father sees the son. The Father is beautiful in His mercy, but the son is also beautiful in the Father's mercy and in himself. Look what the Father does: he puts a ring on the finger of his son, he puts a new cloak around his shoulders, he puts new sandals on his feet, he kills the fatten calf and celebrates with song and dance. The son did not know his own dignity, but the Father celebrates the son's beauty and dignity, as the son mumbles rather imperfectly a confession of his sins. Mercy is the communion of two beautiful persons. When we look at the appearances of Jesus to the disciples after the His resurrection, we see again the merciful love of Christ for his fallen and doubting disciples. When the Risen Lord, for example, looks on Peter, he does not see him as a "ruined apostle" but as a beautiful shepherd of the sheep.
When we come to the Lord, and seek His pardon, He looks upon us with a gaze of wonder, because He can see our deep down beauty, even if hidden by our deep down sufferings and sins. And when He looks upon us something of His glory shines upon our hearts and faces. We must not think that Peter receives a different kind of mercy than we do. No, Peter, Thomas, the Eleven, the Disciples of Emmaus....all received mercy, the same kind that we receive. It is a mercy that allows our beauty to shine, that allows God's glory to radiate in us, that allows us to find a new place for showing mercy in our hearts to others who do not realize yet how beautiful they are.