"O praise the LORD, all ye nations: praise him, all ye people. For his merciful kindness is great toward us: and the truth of the Lord endureth for ever. Praise ye the LORD." (Psalm 117:1-2)
Psalm 117 is especially noteworthy for two reasons: first, it is the middle chapter of the Bible, and, secondly, it is the shortest chapter in the Bible, consisting of only the two verses cited above. Thus, it is significant and appropriate that its theme be that of universal and everlasting praise. The very purpose of human language is that God might communicate His word to us, and that we might respond in praise to Him.
The word "nations" in verse 1 refers specifically to Gentiles, while "people" seems to refer to all tribes of people. Two different Hebrew words for praise are used, so that the verse could be read: "Praise the LORD, all ye Gentile nations; extol him all ye peoples of every tribe." In any case, the sense of the exhortation is to urge everyone to praise His name.
The Hebrew word translated "merciful kindness" is also rendered as "loving kindness," or simply "mercy," or "kindness." Whichever is preferred, the significant point is that it has been great toward us. This word (Hebrew, gabar) is not the usual word for "great," but is a very strong word meaning to "triumph" or "prevail." An example of its use is in the story of the great Flood. "And the waters prevailed exceedingly upon the earth" (Genesis 7:19). In fact, it is used four times in this account of the "overwhelmingly mighty" waters of the Flood (Genesis 7:18-20, 24).
In other words, God's merciful kindness has prevailed over our sin and the awful judgment we deserve in a manner and degree analogous to the way in which the deluge waters prevailed over the ancient evil world.
God's mercy and truth are eternal, and this will be the great theme of our praise throughout all the ages to come. HMM