For the key which binds carries forward the work of the law. It is profitable to the sinner inasmuch as it reveals to him his sins, admonishes him to fear God, causes him to tremble, and moves him to repentance, and not to destruction.
The loosing key carries forward the work of the gospel. It invites to grace and mercy. It comforts and promises life and salvation through the forgiveness of sins.
In short, the two keys advance and foster the gospel by simply proclaiming these two things: repentance and forgiveness of sins [Luke 24:47]. -- Blessed Martin Luther, *On the Keys* AE 40