John 16: 8-9 When he comes, he will convict the world of guilt in regard to sin and righteousness and judgment: in regard to sin, because men do not believe in me.
1 Cor 14:24 But if an unbeliever or someone who does not understand comes in while everybody is prophesying, he will be convinced by all that he is a sinner and will be judged by all. (NIV)
For the unbeliever prophecy brings rebuke and conviction of sin as the Holy Spirit acts on the unrepentant heart with the hammer blows of the Law. We too bring the Law - the glory due a holy and righteous God in whose sight sinners cannot sand without the secrets of the heart being laid bare (1 Cor 14:25). AS it brings conviction, the gift of prophecy evokes God's holy presence, sows the seeds of repentance as they come into the throne room of our Lord whose judgment flashes like lightning (Hosea 6:5). Their tongues falling silent before his holy Law (Romans 3:19).
Used in this way, prophecy produces a healthy fear of God. From which the soul is led to Christ, being made conscious of sin (Romans 3:20), and feeling its need for forgiveness which our Lord is quick to bestow.
The gift of prophecy then is the application of Law and Gospel (although I'm willing to concede that it has other important functions). It brings revelation that under God's Law we all stand guilty of transgression. And the revelation of Jesus' testimony through which the forgiveness of sins, and the justification by faith alone, is offered to those who believe. When applied to our lives, the prophetic should offer the severity of God's law to those secure in their sins, and the sweetness of the Gospel to those in terror of the law. All in step with the Holy Spirit whose primary purpose is to bring us to Christ.