If you’ve never read Ministry, Word, and Sacraments: An Enchiridion by Martin Chemnitz, I suggest you get one pronto, and start reading Chemnitz on the Keys, Absolution, and the Ministry (among other things). Here is a (long) excerpt from his section on Absolution:
What does Christ call the keys of the kingdom of heaven?
The ministry of the Word itself, in which and through which, by divine command and power, sins are bound and retained to the impenitent and unbelieving (Mt 16:19; Jn 20:22-23).
Why does Christ attach to the ministry the name keys of the kingdom of heaven?
To indicate that the preaching of the Word is not a vain and useless babbling of words, but that the Holy Spirit is present in this ministry, is efficacious through it, and wants, by this means as with a kind of keys, the kingdom of heaven to be unlocked and many to be brought into it. Therefore ministers of the Word should, by this term, since they hear that the keys of the kingdom of heaven have been given and entrusted to them, bestir themselves to speak the doctrine of sin, repentance, faith, forgiveness of sins, new obedience, etc., not coldly and lightly, nor only in general or superficially, as a story, but set it forth and apply it to their hearers faithfully and diligently, sure that the Holy Spirit wants to kindle, increase, strengthen, and preserve repentance, faith, and new obedience in the hearts of the hearers through this their ministry. This name also encourages their spirits, that their labor is not in vain in the Lord (1 Co 15:58). But if ever in the ministry they bind and retain sins to the impenitent, according to the Word of God, with threat of divine wrath and curse, they should know that this is regarded as valid and certain also in heaven. In the same way, if they loose and forgive sins by proclaiming the grace of God to the penitent and believing, they should be sure that it is not only good words (as is commonly said), but that the same is also certain and confirmed in heaven (Mt 16:19). Similarly the name keys should admonish the hearers not to despise the Word and ministry and regard it as a vain sound of words by which only the ears are struck, but that they might know and be firmly persuaded that if they want to enter the kingdom of heaven, the approach and entrance is not given and granted to them except through these keys. And if they notice that something is lacking in repentance, faith, new obedience, etc. in themselves, they should remember that the Holy Spirit wants to supply and increase the things that are lacking in them by this means and instrument. So also, when they are censured and rebuked by the exhortation of the Law, let them not think that they are vain warnings that can be shaken off, like dust from a garment; but let them know, if they continue in impenitence and unbelief, that whatever is bound in this way on earth, the same is also bound and retained in heaven. On the other hand, when they hear the promises of the Gospel set forth for believers on the basis of the Word of God, let them not think that they are only words, and that of a man; but let them consider and firmly believe that it is the voice of the Holy Spirit Himself, bringing comfort to our hearts through this means, and that whatever is loosed on earth in that way is loosed and forgiven in heaven by God Himself, because it is done through the keys of the kingdom of heaven. It should therefore be earnestly set forth and often considered by Christ wanted to speak in this way about the ministry: “I will give unto thee the keys of the kingdom of heaven.”
Ministry, Word, and Sacraments: An Enchiridion, pp. 132-133.