The psalmist knows – compared to the word of the Father everything else pales in comparison.
The world and everything in it will pass, but God’s word will stand. It gives guidance and protects us from making bad decisions. It gives wisdom beyond our human ability and understanding.
It takes faith to trust in God’s word because many times it goes against our understanding. But the more we obey in faith, the more God will show that He is true. I think that’s what happened to the psalmist, the psalmist knows because he has experienced the love and the power (and the power of the love!) of God.
Time for personal application.