Albert Barnes Commentary Psalms 136:2

"Oh give thanks unto the God of gods; For his lovingkindness [endureth] for ever." — Psalms 136:2 (ASV)

O give thanks unto the God of gods - See (Deuteronomy 10:17). The supreme God; the God superior to all that is called God, or that is adored by the nations of the earth; above all to whom the name God is ever applied.

For his mercy... - The ground of praise here is that it is a characteristic of the supreme God that he is a merciful Being; that there is blended in his character eternal mercy with infinite power. Mere power might fill us with dread; power mingled with mercy, and able to carry out the purposes of mercy, must lay the foundation for praise.