Bless the Lord, O my soul, and all that is within me, bless his holy name! (Psalm 103:1)
The psalm begins and ends with the psalmist preaching to his soul to bless the Lord — “Bless the Lord, O my soul” — and preaching to the angels and the hosts of heaven and the works of God’s hands that they should do the same.
Bless the Lord, O you his angels,
you mighty ones who do his word,
obeying the voice of his word!
Bless the Lord, all his hosts,
his ministers, who do his will!
Bless the Lord, all his works,
in all places of his dominion.
Bless the Lord, O my soul!
The psalm is overwhelmingly focused on blessing the Lord. What does it mean to bless the Lord?
It means to speak well of his greatness and goodness — and really mean it from the depths of your soul. What David is doing in the first and last verses of this psalm, when he says, “Bless the Lord, O my soul,” is saying that authentic speaking about God’s goodness and greatness must come from the soul.
Blessing God with the mouth without the soul would be hypocrisy. Jesus said, “This people honors me with their lips, but their heart is far from me” (Matthew 15:8). David knows that danger, and he is preaching to himself. He is telling his soul not to let this happen.
“Come, soul, look at the greatness and goodness of God. Join my mouth, and let us bless the Lord with our whole being. Soul, we are not going to be a hypocrite!”
Source: John Piper https://www.desiringgod.org/articles/what-it-means-to-bless-the-lord