"I will greatly rejoice in the Lord ...
for He has clothed me
with the garments of salvation." (Isaiah 61:l0)
At no point in our worship should we do things thoughtlessly, simply "going through the motions." This is especially true in our singing. "Sing praises with understanding" (Ps. 47:7). "I will sing with the spirit [i.e., in the power of the Holy Spirit, filled with the Holy Spirit, from a heart made new by the Holy Spirit], and I will also sing with the understanding" (1 Cor. 14:15). These are standing orders for the church in every age!
It will help you to "sing with the understanding" when you consider the place and the purpose of each hymn (or psalm) in the liturgy, that is, the order of worship. In the first hymn, we approach God in praise for who he is and what he does or has done. In other hymns, we prepare for the ministry of the Word or respond to it. After our corporate confession of sin and the assurance of God's pardon, we respond with thanks for the amazing grace of God. God's person and works prompt us to praise, but his forgiving mercies toward us prompt our special praise.
Think of this time as your response to receiving a new suit of clothes. You come before God "wretched, miserable, poor, ... and naked." (If you think that's an overstatement, read Revelation 3:17!) You confess all of that as you are led by the minister in the corporate confession of sin. Then, in the assurance of pardon, God, through the work of his Son, provides you with "white garments, that you may be clothed, that the shame of your nakedness may not be revealed" (Rev. 3:18). By grace, through faith, you have received a new suit of clothes!
How do you feel when you have gone from "filthy rags" (Isa. 64:6) to fresh, new, beautiful apparel? There is happiness and joy! You want to go out and show the world what you have received!
That's the spirit of the response of praise following the assurance of pardon. It ought to be the most intensely personal and joyous expression of praise in an entire worship service, because it expresses the most intensely personal and joyous blessing of the grace of God!