psalm 136Psalm 136  – 1-3     Oh, give thanks to the Lord, for He is good!
For His mercy endures forever.
2 Oh, give thanks to the God of gods!
For His mercy endures forever.
3 Oh, give thanks to the Lord of lords!
For His mercy endures forever:

Luther says,

The 136th psalm is a psalm of thanks and is perhaps the text to show priests how they should sing and preach. Namely, they should sing and preach of God and His wonderful deeds, that He is gracious and merciful and a true Savior. Therefore, in each verse the psalmist repeats the line, “His steadfast love endures forever,” with which the psalm is nearly overwhelmed. Truly nothing but grace, not human works or doctrines, should ever be preached. For human works and words have done no wonders such as these. They are not deserving of this worship, but rather only the grace and pure goodness of Him who gives all. Gift! Gift, he says! Gift—free of charge! And Christ also stands hidden in the phrase. Such doctrine keeps in the people a pure faith and a right understanding of grace and the forgiveness of sins, against the rebellious and stiff-necked work-saints