Albert Barnes Commentary Psalms 136:26

"Oh give thanks unto the God of heaven; For his lovingkindness [endureth] for ever." — Psalms 136:26 (ASV)

O give thanks to the God of heaven—The God who reigns in heaven; whose home is heaven.

For his mercy ...—In view of all this—of all that he does in heaven and on earth—let praise be ascribed to him. To know the measure of the praise due to him, and to see how great his mercy is, it would be necessary to know all that he does in heaven and on earth.

That will not be known here. It will constitute the theme of contemplation and praise forever and ever.

Enough, however, is known here to show the propriety of repeating again, again, and again, as in this psalm, the language: For his mercy endureth forever; For his mercy endureth forever; For his mercy endureth forever.