It seems to me that both the words of Jesus ("forgive them for they know not what they do" etc.) and the teachings of the apostles ("he died for our sins" etc.) confirms the notion that forgiveness was actualized in the cross of Christ. As a fan of the resurrection, I understand the temptation of those who might recoil from this cross-centered "atonement" theory. But it seems to make sense to me, so I'm not going to give up on it just yet.
Hence the sin of Peter, his denial, is in a certain sense already forgiven on the cross. But he doesn't know it yet, and so it does not yet make a difference in his life.
For what we do not yet have in the cross of Christ is a restored fellowship with God. The barrier to fellowship, i.e., our sin, has been removed. Hence the cross is the decisive event, the turning point in the story of God. But the aim or purpose of removing our iniquities is to welcome us into fellowship with God.
And this restoration takes place in Christ's resurrection.
Objectively, Jesus is our elder brother, the firstborn from the dead, whom the Father has welcomed into eternal living fellowship by raising him from the dead. Subjectively, Jesus comes to his own, breaks bread with them, and welcomes them into his fellowship so and also into fellowship with his eternal Father. He comes to Peter, puts the question of love to him, and commissions him for his missionary task. He restores fellowship with Peter, and sends him to the ends of the earth as an ambassador of the reconciliation achieved in the cross-and-resurrection of Jesus.
So, in a word, God forgives our sins in the cross of Christ and restores us to fellowship in the resurrection of Christ.
Perhaps too clean and simple. But I think that gets at least some of what needs to be said about the good news of the forty days!