Ps. 136 has been known among the Jews as the Great Hallel. The recurring refrain “for his mercy endureth for ever” was doubtless sung as a response, either by the worshipers or by a Temple choir.
His mercy endureth for ever.
Literally, “for to eternity his love.” The word translated “mercy” is chesed, which means, “divine love” (see Additional Note on Ps. 36). The refrain is repeated in every verse of this song.
O give thanks.
There is no time when it is inappropriate to render thanks to God for His goodness and mercy to man.