"If you forgive the sins of any, their sins have been forgiven them; if you retain the sins of any, they have been retained" (John 20:23).
Perhaps an underlying assumption in this text is that should we ever retain the Sins of any we ourselves have retained our own Sins. In other words, when we do not forgive we ourselves remain unforgiven.
I often find that I have been formed to think of texts like this in terms of my responsibility to decipher which sins people commit ought to be forgiven and when. That’s just silly. Sin is bigger than that. Consider the full reality of Sin as that of a complex network of bad fellowship (physiological, emotional, physical) in which we are both victim and perpetrator of Sin in both intentional and unintentional ways. In this verse the risen Lord standing in the midst of His people breathing the Holy Spirit on them, the same Holy Spirit the empowered Jesus to fulfill all righteousness, or covenant faithfulness (both from God to humanity and from humanity to God), through whom we have the forgiveness of Sins, that is, through whom we have been redeemed, brought out, liberated from the power of Sin through this One who both effects and demonstrates our salvation.
To retain the Sins of others is to leave them still within this complex network of bad fellowship. It seems that Jesus is warning His people of the responsibility they now have to be a people of atonement, a people who forgive the Sins of many just like Jesus. Our responsibility is not to decipher when to retain forgiveness. Rather it is to be a people who live according to the rule and reign of God (good fellowship) in such a way that other can become incorporated into the body of Christ where there is forgiveness and redemption.