Psalm 98 depicts God’s salvation and righteousness. He has not forgot his steadfast love and faithfulness, something that was called into question back in psalms 88-89. Here this Psalm speaks of triumph. 

God’s people then sing a psalm and join in on the victory that God their King has won for them. Worship then, is always an anticipation of the victory of Christ. 

Even the whole creation joined in the praise as the hope of the psalm is made clear. The Lord comes. The Lord will come to judge. This is the victory that we sing. God’s judgment will prevail and God will act in history to achieve this. And God has through the work of his Son, Jesus Christ.