Psalm 33 opens with a call: rejoice in the Lord, O you righteous, for praise is fitting for the upright. The first part of the lesson dwells on that call, God summoning His people to praise Him and to sing a new song, the song of the redeemed in Revelation 14. It separates happiness, which depends on circumstances, from joy, which can be present in trials, persecution and even death because it rests on what God has done for us in Christ. There is no true joy without the peace of God.
The second part takes up the reason for that praise: the word of the Lord is right and all His work is done in truth. Scripture is God’s revealed word, pure, sufficient and the power of the gospel for salvation. His works in creation, providence and redemption show His faithfulness. And because He loves righteousness and justice, His people are called to do them.