The Office of the Keys is that special authority which Christ has given to His church on earth to forgive the sins of repentant sinners, but to withhold forgiveness from the unrepentant as long as they do not repent.
This is what St. John the Evangelist writes in chapter twenty: The Lord Jesus breathed on His disciples and said, “Receive the Holy Spirit. If you forgive anyone his sins, they are forgiven; if you do not forgive them, they are not forgiven.” (John 20:22–23)
I believe that when the called ministers of Christ deal with us by His divine command, in particular when they exclude openly unrepentant sinners from the Christian congregation and absolve those who repent of their sins and want to do better, this is just as valid and certain, even in heaven, as if Christ our dear Lord dealt with us Himself.
*This question may not have been composed by Luther himself but reflects his teaching and was included in editions of the catechism during his lifetime.