[1] Praise the LORD, all nations! Extol him, all peoples!
[2] For great is his mercy toward us; and the faithfulness of the LORD endures for ever.
Praise the LORD!   (Psalm 117)

One of the main characteristics of the Psalms is that they use a literary technique called “parallelism”.  Verse 1 in Psalm 117 is a perfect example of this. Praise and Extol are synonyms, and all nations / all peoples mean the same thing as well.  So when reading or praying with the Psalms be aware of and look for the parallelism.

Some of the Psalms are also a prophecy, Verse 1 in its demand for all the peoples, of all the nations to praise the Lord is such a prophecy.  When written 3000 years ago the only nation and the only people who were praising the Lord was the Jewish nation.  Today, many more people are praising the Lord, perhaps 2 billion Christians, but still the entire world is not joining us, and so the prophecy has not yet come true. The question must be asked: What is our role in this? I think we need to be praising the Lord publicly so that the rest of the nations and peoples can see our example and someday join us.

In verse 2 the writer is stating a truth about the Lord’s mercy, but it is a special truth that finds its fulfillment, or its complete understanding in the future.  The writer of the psalm knew the Lord as One, we however, through the teaching of Jesus, know the Lord as a Trinity. Who is it in the Trinity that is merciful to us? Jesus.  So we can start to understand this verse by making a small change:
For great is the mercy of Jesus toward us.
Now the next step, we can get the full understanding of this verse:
For great is the divine mercy flowing from the Sacred Heart of Jesus toward us.

Finally, what is meant by the faithfulness of the Lord? The faithfulness of the Lord is not a “concept” it is a reality, a reality the the Lord has shown and continues to show us. What is the Lord faithful about? His love for us. The best examples of this faithfulness of the Lord is expressed in the divine promises of the covenants, starting with Adam, Noah, Abraham, Moses, David and finally with Jesus. Matthew chapter 26:28
“for this is my blood of the covenant, which is poured out for many for the forgiveness of sins.” That is the Lord’s faithfulness.

To summarize, this Psalm is a prophecy that finds it fulfillment in a person, for mercy and faithfulness have a name: Jesus.