"Then said Jesus to them again, Peace be unto you: as My Father hath sent Me, even so send I you. And when He had said this, He breathed on them, and saith unto them, Receive ye the Holy Ghost: Whose soever sins ye remit, they are remitted unto them; and whose soever sins ye retain, they are retained." John 20:21-23
On the evening of that first Easter Day, Jesus appeared to His disciples where they were gathered together behind closed doors for fear of the Jews and showed them His hands and His side to prove to them that He truly was Jesus and that He really had risen from the dead (John 20:19-20).
Two times Jesus said to His disciples: "Peace be unto you." Since Christ had died on the cross for their sins and was risen again in victory, they had peace with God -- the peace of sins pardoned and forgiven through the shed blood of Jesus (cf. Eph. 2:11ff.; 1 John 2:1-2).
The Bible says: "Therefore being justified by faith, we have peace with God through our Lord Jesus Christ: by whom also we have access by faith into this grace wherein we stand, and rejoice in hope of the glory of God" (Rom. 5:1-2). We too, when we acknowledge our sins and look in faith to Jesus, have forgiveness of sins and peace with God! And, we too look forward to the glories of life eternal in heaven for Jesus' sake!
Not only did Jesus bless His disciples with the peace of sins forgiven, He breathed on them, gave them His Holy Spirit and commissioned them to forgive the sins of penitent sinners and to retain (not forgive) the sins of impenitent sinners as long as they do not repent (John 20:21-23).
This is often spoken of as the Office of the Keys because it opens the gates of heaven to those who are sorry for their sins and look in faith to Christ and His redemptive work and closes the gates of heaven to those who are not sorry for their sins or do not trust in Christ for forgiveness.
And, indeed, it takes the wisdom and guidance of the Holy Spirit to rightly use and apply these keys to ourselves and to others, but this is what Christ our Savior would have us, as believers and disciples of Jesus, do!
Some would object and say, "How can we, as believers in Christ, forgive and retain sins? Only God can do that!" That is true but this power is given to us as believers and a part of Christ's Church precisely because of Christ's death on the cross for the sins of all mankind and His glorious resurrection on the third day.
Since God forgives the sins of penitent sinners who look in faith to Christ and His cross, we can and should proclaim forgiveness to penitent sinners who look to Christ for pardon and forgiveness. And, since God retains the sins of the impenitent and unbelieving, we too can and should proclaim to the impenitent and unbelieving that their sins are not forgiven until they repent and look in faith to Christ.
Therefore, because Christ has paid for all sin and is risen in victory, we announce and proclaim God's pardon and peace to penitent sinners - to those who acknowledge and repent of their sins and look in faith to Christ and His cross for pardon and forgiveness. And because Christ is the only way of salvation, we tell those who continue in sin and unbelief that they remain dead in their sins and we warn them of the coming judgment of God.