"Woe is me, for I am undone! Because I am a man of unclean lips, and I dwell in the midst of a people of unclean lips; for my eyes have seen the King, the Lord of hosts." (Isa. 6:5)
All worship has three basic parts. In the first, we consciously and thoughtfully come before the Lord, knowing that he promises to be with us as we are gathered in his name (see Matt. 18:20). In the third (and, by far, the longest part of worship), we sit in the presence of God and hear his Word read and preached. Before we can do that, however, there is the all-important "middle" part of worship: our confession of sin.
In the Bible, confession of human sin, failing, and weakness is the invariable response when people come into the presence of God. After God describes and displays his glory to Job, Job's response is, "I have heard of You by the hearing of the ear, but now my eye sees you. Therefore I abhor myself, and repent in dust and ashes" (Job 42:5-6). And, lest we think that this dynamic changes in the New Testament, when the apostle John found himself in the presence of the exalted Jesus Christ, the God-man, he "fell at His feet as dead" (Rev. 1:17).
The same pattern is found in Isaiah 6. Isaiah is in the presence of the holy God (vss. 1-4). His response is a heartfelt confession of his own sin and of the sin of his people (vs. 5). This confession and an object lesson in forgiveness of sins (vss. 6-7) precede Isaiah's reception of the word of God from the Lord's mouth (vss. 8-13).
Our sinful human nature demands this order when we know that we are in the presence of God. We must be cleansed from our sins before we are fit to receive God's word for the good of our souls. In the corporate confession of sin in public worship, we are led by the minister (who, at that point, speaks on behalf of the congregation to God) in confessing our sins before a holy God and begging for his mercy on us. How thankful we should be that there is forgiveness of sins in Jesus Christ, so that we might worship and serve God with clear consciences and pure hearts (Ps. 130:4).