And when he had said this, he breathed on them and said to them, "Receive the Holy Spirit. If you forgive the sins of any, they are forgiven them; if you withhold forgiveness from any, it is withheld" (John 20:22-23, ESV).
These verses link together the gift of the Spirit to the church with the authority of the church to pronounce on whether someone's sins are forgiven.
What on earth is this about?
Some might take these words to mean that, when a Christian commits a sufficiently serious sin, they absolutely must make confession of that sin to a minister of the church, otherwise that sin will not be forgiven. However, these words of Jesus (and similar words elsewhere) are certainly not sufficient to establish such a practice, nor do we read of such a thing happening anywhere in the New Testament. Now, it may well be valuable for us to confess our sins to one another, and to assure one other of Christ's forgiveness. But it is difficult to see that as the primary meaning of these words.
Others might take them to mean this, and nothing more than this: that the disciples were commissioned to proclaim that whoever believes in Jesus will be forgiven, and whoever does not believe will not be forgiven. But, if this was all that Jesus intended to convey here, it is difficult to see why he didn't express it more clearly. His words seem much more specific, and seem to give the disciples the role of saying of particular people, that their sins are forgiven, and of other particular people, that their sins are not forgiven.
So what do these words mean?
Jesus' words are a promise. Jesus promises that there will be a correspondence between what the disciples say is the case (in terms of forgiveness) and what is actually the case (in terms of forgiveness). It seems to me that what Jesus is promising here is precisely what we find happening in the book of Acts. On the day of Pentecost, Peter said, "Repent and be baptized every one of you in the name of Jesus Christ for the forgiveness of your sins, and you will receive the gift of the Holy Spirit" (Acts 2:38, ESV), and then we read of 3000 who were added to their number that day. Presumably, given Peter's words, those 3000 were baptised, and their baptism was taken to be a sign that their sins had been forgiven. So Jesus' promise would mean that this sign of forgiveness was not an empty sign, but that the sins of those 3000 people really had been forgiven.
It seems to be the case that, in Acts, people responded very clearly and powerfully to the proclaimed word. People heard the word, responded with faith, received the Holy Spirit, and showed undeniable evidence of that. It was abundantly clear that some people had received forgiveness, and that other people had not received forgiveness. (Take Cornelius and his companions for an example of the former.) Jesus' words, therefore, I take to be a promise that the initial preaching of his word would be accompanied with a powerful work of his Spirit, such that the disciples would be able to declare accurately and with conviction that the sins of certain people had been forgiven, and that the sins of certain other people had not been forgiven.
What about today?
It should be clear that things are not always so clear! There are many in the church, who have received the sign of forgiveness in baptism, who show no evidence of the work of the Holy Spirit in their lives, and who therefore show no evidence that they really have been forgiven. We still administer the sign of baptism, on a credible profession of faith, but it can be many years after receiving someone into the church that it becomes undeniable that the outward sign has been accompanied by an inner transformation (or not!). So might it be the case that, in building his church, there are times when Jesus makes his true church abundantly visible, and times when he allows his true church to be somewhat hidden from view? Certainly in the apostolic age the true church was made powerfully visible. And I'm sure that is also the case today in other situations of persecution or of mission or of revival. But that is not necessarily the case in every time and place.
Perhaps we should be laying hold of Jesus' promise, and praying that, by a work of his Spirit, his true church may become more visible in our own day? The whole creation is longing for such a day, and shouldn't we? "For the creation waits with eager longing for the revealing of the sons of God" (Romans 8:19, ESV).