“Great is our Lord, and abundant in power; his understanding is beyond measure” (v5). The psalmist worshipped by declaring the specifics of God’s character and actions. He “determines the number of the stars … He prepares rain for the earth; he makes grass grow on the hills” (v4,8). We don’t give glory to an unknown Power; we give praise to our kind Shepherd who “heals the brokenhearted and binds up their wounds (v3). We “hope in his steadfast love” (v11) because He has demonstrated His love to us.
Worshipping and testifying are ultimately the same thing: when we give glory to God for His wonderful works, we declare to the world who He is and what He has done.