Matthew Henry on Psalms 136:1-9

Forgetful as we are, things must be often repeated to us. By “mercy” we understand the Lord's disposition to save those whom sin has rendered miserable and vile, and all the provision he has made for the redemption of sinners by Jesus Christ. The counsels of this mercy have been from everlasting, and the effects of it will endure for ever, to all who are interested in it. The Lord continues equally ready to show mercy to all who seek for it, and this is the source of all our hope and comfort.