Charles H. Spurgeon on Psalms 136:15

But overthrew Pharaoh, etc. I know that the Gospel is a book of mercy; I know likewise that in the prophets there are many expressions of mercy; I know likewise that in the ten commandments, which are the ministration of death, there is made express mention of mercy, "I will have mercy on thousands": yet, notwithstanding all this, if every leaf, and every line, and every word in the Bible were nothing but mercy, it would nothing avail the presumptuous sinner. Our God is not an impotent God with one arm; but as he is slow to anger, so is he great in power. And therefore though in this Psalm there is nothing but his mercy endureth for ever, which is twenty-six times in twenty-six verses: yet mark what a rattling thunder clap is here in this verse. In our addresses therefore unto God, let us so look upon him as a just God, as well as a merciful; and not either despair of or presume upon his mercy. —Abraham Wright.