Commentary on the Bible, by Adam Clarke, , at sacred-texts.com
The psalmist calls upon the nations of the world to praise the Lord for his mercy and kindness, and for the fulfillment of his promises, Psa 117:1, Psa 117:2.
This is the shortest Psalm in the whole collection; it is written as a part of the preceding in thirty-two of Kennicott's and De Rossi's MSS., and is found thus printed in some ancient editions. The whole Psalm is omitted in one of Kennicott's and in two of De Rossi's MSS. It celebrates the redemption from the Babylonish captivity, the grand type of the redemption of the world by our Lord Jesus.
The Syriac says: "It was spoken concerning Ananias and his followers when they came out of the furnace; but it also foretells the vocation of the Gentiles by the preaching of the Gospel." In this way St. Paul applies it, Rom 15:11.
O praise the Lord, all ye nations - Let all the Gentiles praise him, for he provides for their eternal salvation.
Praise him, all ye people - All ye Jews, praise him; for ye have long been his peculiar people. And while he sends his Son to be a light to the Gentiles, he sends him also to be the glory of his people Israel.
For his merciful kindness is great - גבר gabar, is strong: it is not only great in bulk or number, but it is powerful; it prevails over sin, Satan, death, and hell.
And the truth of the Lord endureth for ever - Whatsoever he has promised, that he will most infallibly fulfill. He has promised to send his Son into the world, and thus he has done. He his promised that he should die for transgressors, and this he did. He has promised to receive all who come unto him through Christ Jesus, and this he invariably does. He has promised that his Gospel shall be preached in every nation, and this he is doing; the truth of the Lord remaineth for ever. Therefore, Praise ye the Lord!