“The word that is used for ‘love’ in this refrain is the powerful Hebrew term hesed, which means ‘covenant love’ or the favor God shows to those with whom He has entered into a covenant relationship. Sometimes it is translated ‘steadfast (or ‘enduring’) love.’ It is enduring because God is a God of His word. He is forever good, and He does not break His covenant.
One night in February 358 A.D. the church father Athanasius held an all-night service at his church in Alexandria, Egypt. He had been leading the fight for the eternal sonship and deity of Jesus Christ, knowing that the survival of Christianity depended on it. He had many enemies—for political even more than theological reasons—and they moved the power of the Roman government against him. That night the church was surrounded by soldiers with drawn swords. People were frightened.
With calm presence of mind Athanasius announced the singing of Psalm 136. The vast congregation responded, thundering forth twenty-six times, ‘His love endures forever.’ When the soldiers burst through the doors they were staggered by the singing. Athanasius kept his place until the congregation was dispersed. Then he too disappeared in the darkness and found refuge with his friends.
Many citizens of Alexandria were killed that night, but the people of Athanasius’s congregation never forgot that although man is evil, God is good. He is superlatively good, and ‘His love endures forever.'”
–James Montgomery Boice, Psalms: An Expositional Commentary, Vol. 3 (Grand Rapids, MI: Baker, 1998), 3: 1185. Boice is commenting on Psalm 136.