Adam Clarke on Psalms 136:1

O give thanks unto the Lord: for he is good - This sentiment often occurs: the goodness of the Divine nature, both as a ground of confidence and of thanksgiving.

For his mercy endureth for ever - These words, which are the burden of every verse, כי לעולם חסדו ki leolam chasdo, might be translated: "For his tender mercy is to the coming age:" meaning, probably, if the Psalm be prophetic, that peculiar display of his compassion, the redemption of the world by the Lord Jesus. These very words were prescribed by David as an acknowledgment, to be used continually in the Divine worship, see 1Chr 16:41 : also by Solomon, 2Chr 7:3, 2Chr 7:6, and observed by Jehoshaphat, 2Chr 20:21; all acknowledging that, however rich in mercy God was to them, the most extensive displays of his goodness were reserved for the age to come; see 1Pet 1:10-12 : "Of which salvation the prophets have inquired, and searched diligently, who prophesied of the grace that should come unto you, - unto whom it was revealed, that not unto themselves, but unto us, they did minister the things which are now reported unto you by them that preached the Gospel unto you by the power of the Holy Ghost sent down from heaven," etc.