"If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness." (1 John 1:9 NKJV)
Reformed worship is marked by its great use of the Word of God. Even the prayers in Reformed worship are formed by the Scriptures as the Word of God. That includes the prayer of confession, which usually follows a reading of Scripture.
The biblical understanding of confession is "to say the same thing as ..." When we confess our faith, we "say the same thing" as God tells us in his Word regarding himself, the way of salvation, and what a true Christian is to be. When we confess our sins, we "say the same thing" as God regarding what sin is, how serious it is, and how much we desire to be delivered from it and its effects. In order to do that, we use the Scriptures as our guide.
In preparing to lead the congregation in corporate prayer of confession of sin, the minister does several things:
How wonderful this portion of worship is! God himself tells us (and in a responsive reading we remind one another) how far short of his glory we fall. In heartfelt prayer before that God, we make a humble confession of our many failings as we are led by the one called to minister on our behalf. And we do all this before "the throne of grace," encouraged by many, many promises of divine forgiveness. Is there any other way to approach the absolutely holy God?