There is certainly a place for preaching to those who are not redeemed, and a very important place, too. But if those are the only people to whom the message is directed, then are the Christians at the service just spectators? Are they there only because they should be? Is the Gospel only for those other people, but not for us who believe?
In the past few years, having been influenced by some people among Lutheran and Reformed churches, and attending an Anglican church, it's been a great relief and blessing to learn that the Gospel is also for me, too.
I am a Christian, but I have also sinned. I have been selfish and self-centered. I have been covetous. I have been angry without good cause, I have said things I should not have said, I have not said things I should have said. Even when I've done things that I would like to think are good works, I must acknowledge the truth of the Bible's statement that all of my works of righteousness are as filthy rags, that they are as soiled and polluted by my sins as a baby's soiled diaper.
In the services at the church I now attend, there is a time when we pray a prayer of confession, where we confess that we have sinned against God in thought, word, and deed, by what we have done and not done. We confess that we have neither loved God with all our hearts, nor our neighbors as ourselves. A bit later in the service, we will celebrate the Lord's Supper.
The Christian guy who's put in 40+ hours the past week, the Christian woman who was waiting tables the evening before, the Christian parents who struggled to get the kids ready for church that morning, the Christian high school student who's struggling with all the things such a student goes through, all of these normal, average Christians, we all need to be reminded that Christ died for us, so that our sins can be forgiven. This isn't to excuse our sins, but to remind us that “Christ Jesus came into the world to save sinners, of whom I am the chief”. While we should be growing in grace and doing the good works God has given us to do, the Gospel reminds us that the Christian life is not about what we do, but about what Christ did for us.
The Gospel is an important message for the unredeemed, that is true, but it is just as important for the Christian, too.