Here’s a excellent and brief explanation by Carl Trueman in his article entitled, “A Word to the Conscience.” It can access the article in its entirety here:
At some point prior to the sermon each Sunday in my church, the minister or elder leading the service will read a passage of Scripture designed to expose the moral failure of fallen humanity before God. Then he will lead the congregation in a corporate prayer of confession. Finally, when he closes the prayer, he will read a short passage (often just a verse or two) which speaks of the forgiveness of sins in Christ. The dramatic theological movement of the service at that point is profound: the congregation goes from being reminded and convicted of their sin, to calling out to God for forgiveness, to being reminded that in Christ God has acted in a startling and decisive way to cast our sin as far away as the east is from the west. We are reminded of the entire gospel, from fall to redemption to consummation, in the space of just a few minutes.
This moment in the church service has come to mean much to me. This is the point where, after a week of failure—of not living up to the standards I set myself, let alone those set for me by my Creator—I am reminded once again that all is well: Christ has dealt with my sin; my failings were placed on his shoulders on the cross; and my heavenly Father has annihilated them there. It is not, of course, that I do not know this Monday to Saturday; it is not that I do not read the gospel every day in my Bible; it is not that I do not confess my sins during the week and look then to Christ. But this is a word from outside, God’s work spoken to me by another human being, which lifts my head once again and assures my conscience that I am clean despite the filth I so often choose to wade in. So often I enter church weighted down with care; when I am once again reminded of God’s rich forgiveness in Christ, the weight is wonderfully lifted from my shoulders.
So often Christians can tend to think of the church worship service as something we do: we sing praise to God; we respond to the gospel; and we rejoice in our Saviour. Further, much discussion in the church focuses on what we need to be doing in order for church to be effective. Yet church is, first and foremost, something which God does. It is primarily and in origin an act of his grace, not an act of human response. He calls us out to be his people; he gathers us through his Spirit; he speaks to us through the reading and the preaching of his word. There is far more passivity in worship than we care to imagine, a passivity that is often belied by our concerns to make sure ‘everybody is involved.’ When the law is read, sins are confessed, and forgiveness declared, we are all involved because we are all included under the words of condemnation and the words of promise and mercy.
– Carl Trueman is Academic Dean, Vice President of Academic Affairs, and Professor of Historical Theology and Church History at Westminster Theological Seminary in Philadelphia, Pennsylvania.