For his merciful kindness is great toward us - His kindness; his compassion; his love. All nations - all people - may say this, and therefore the psalm is adapted to universal praise. Especially may this be said in view of the love of God to mankind in the gift of a Saviour - a Saviour not for any one people especially or exclusively, but for the world, John 3:16.
And the truth of the Lord endureth for ever - All that God has said: his declarations; his promises; his assurances of mercy. They are the same in all lands where they are made known, and they are the same in all ages of the world. Truth is a representation of things as they are; and truth, therefore, must be ever the same. What was true in the first ages of the world in regard to the relation of the sum of the squares on the two sides of a right-angled triangle to the square of the hypothenuse is true now, and will always be true; and so, what God has affirmed at any one time will always remain the same in all ages and in all lands. What was truth to Abraham is truth to us; what was truth to Paul is truth to us; what was truth to the martyrs is truth to us; what is truth to us will be truth to all generations of the world in all lands, and will be truth forever. This fact, too, is a just foundation for universal praise, and therefore the psalm is so adapted to be used in all lands and among all people. How often in our own language has this psalm been the medium of the utterances of praise in Christian sanctuaries:
"From all that dwell below the skies,
Let the Creator's praise arise;
Let the Redeemer's name be sung,
Through every land, by every tongue.
Eternal are thy mercies, Lord;
Eternal truth attends thy word;
Thy praise shall sound from shore to shore,
Till suns shall rise and set no more.

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!

For his merciful kindness is great toward us: and the (a) truth of the LORD [endureth] for ever. Praise ye the LORD.
(a) That is, the most certain and continual testimony of his fatherly graces.

For his merciful kindness is great towards us,.... Not us Israelites only, of whom David was, but Gentiles also; or otherwise there would be no force in the reason why all people and nations should praise the Lord: but it respects the time when these should become one people, partaking of the same grace, blessings, promises, and privileges; in which the grace, mercy, and lovingkindness of God, greatly appeared; "it prevailed over us" (p), as it may be rendered: the word is used of the prevailing of the waters of the flood over the earth, Genesis 7:18; and so may denote the exuberance of the grace of God, of the abounding and superabounding of it. There is an abundance of it in the heart of God, in his covenant, and in his Son; and which is displayed in redemption by him; in the forgiveness of sin; and in the conversion of sinners, and their everlasting salvation: particularly there has been an inundation or deluge of it in the Gentile world, where it has flowed and overflowed; where sin abounded, grace has much more abounded; and therefore the Lord is to be praised. And another reason follows,
and the truth of the Lord endureth for ever; the faithfulness of God to his promises, not only made to the Jewish fathers concerning the Messiah, and redemption by him; but to the Gentiles, and concerning the blessing of all nations in the promised seed: and the faithfulness and truth of God, with respect to any of his promises, never fails; nor will his word of truth, the Gospel; nor Jesus Christ, who is the truth, and the truth of God; the truth of types, promises, and prophecies; see Psalm 43:3; for he is the same today, yesterday, and for ever.
Praise ye the Lord; for his superabounding grace, and eternal truth; even all the people of God, of all nations, Jews and Gentiles.
(p) "exsuperavit", Vatablus; "invaluit", Junius & Tremellius, Piscator, Gejerus, Michaelis; "exuberavit", Cocceius.

This may be regarded as a doxology, suitable to be appended to any Psalm of similar character, and prophetical of the prevalence of God's grace in the world, in which aspect Paul quotes it (Romans 15:11; compare Psalm 47:2; Psalm 66:8). (Psalm 117:1-2)
is great toward us--literally, "prevailed over" or "protected us."