Charles H. Spurgeon on Psalms 136:1

His mercy endureth for ever. It is everlasting. Everlastingness, or eternity, is a perfect possession, all at once, of an endless life (saith Boethius). Everlasting mercy, then, is perfect mercy, which shuts out all the imperfections of time, beginning, end, succession, and such is God's mercy. First, his essential mercy is eternity itself; for it is himself, and God hath not, but is, things. He is beginning, end, being; and that which is of himself and even himself is eternity itself. Secondly, his relative mercy (which respects us, and makes impression on us), is everlasting, too, in a sense; for the creatures, ever since they had being in him, or existence in their natural causes, ever did and ever will need mercy, either preserving or conserving. Preventing or continuing mercy in the first sense is negatively endless, that is, incapable of end, because unboundable for being: in the second sense, it is privatively endless, it shall never actually take end, though in itself it may be, and in some ways is, bounded; the first is included in the latter, but the latter is chiefly here intended; and therefore the point arises to be this, —God's mercy (chiefly to his church) is an endless mercy; it knows no end, receives no interruption. Reasons hereof from the word are these (for as touching testimony this Psalm shall be our security), first, from God's nature, "he is good". Mercy pleaseth him. It is no trouble for him to exercise mercy. It is his delight: we are never weary of receiving, therefore he cannot be of giving; for it is a more blessed thing to give than to receive; so God takes more content in the one than we in the other. —Robert Harris, 1578-1658.