The tenth and eleventh points summarizing my longer explanation of the PMW both clarify what the PMW means in relation to the keys. Here they are:
(10) When the statement speaks of an individual retaining or binding sins privately or unofficially, it does not mean that the individual is bespeaking a sinner to be cut off from heaven in the manner of an excommunication from the Church. Rather, the statement refers to an individual’s authority to repeat the judgments of God upon sin, and so admonish other sinners.
By this I don’t mean that the Law, applied to sinners, ever fails to slay us and condemn our guilt. The Law kills every time it’s applied, even if it’s spoken “in the third use,” as we say.
However, the words “retain” and “bind” in this context imply to many the act of excommunication. It must be clarified that this is quite different from a simple private or unofficial admonishment, though both are expressions of the Law.
(11) When the statement speaks of Christians using the keys to judge the teachings of their pastors and teachers, it only means those pastors and teachers still living on earth, and only those times when the Christian confronts the pastor/teacher with the sin of teaching false doctrine. The Christian’s duty to test the spirits is not an exercise of the keys.
It seems that this should be a no-brainer, but the way the PMW is worded might allow someone to think that the Christian’s duty to test the spirits is an exercise of the keys. It’s not. The Christian receives both the duty and the keys upon entering the Kingdom of Grace, but they are not the same thing. The keys are about sin. Judging false teaching is about doctrine. Though false doctrine happens to be sin, the keys are only in use when God’s Word is applied to the sinner. Naturally, sinners who are elsewhere or who have already died can’t have Law and Gospel applied to them.