Passage
Let Israel now say, That his mercy endureth for euer.
Let Israel now say, That his mercy endureth for euer.
Psalms 118:1 Praise yee the Lord, because he is good: for his mercie endureth for euer.
Psalms 118:2 Let Israel now say, That his mercy endureth for euer.
Psalms 118:3 Let the house of Aaron nowe say, That his mercy endureth for euer.
Psalms 118:4 Let them, that feare the Lord, nowe say, That his mercie endureth for euer.
The verse centers on "mercy", "israel", "endureth", and "euer". It is saying that the verse draws attention to "mercy" and "israel", so its meaning should be read from those terms before moving to application.
The nearby context moves from verse 1's "Praise yee the Lord because he is..." into verse 3's "Let the house of Aaron nowe say...", so "mercy" and "israel" belong inside that flow. In Psalms context, the local focus is worship, trust, the LORD's kingship, and covenant mercy.
A plain takeaway is to answer the verse's own emphasis on "mercy" and "israel" with trust shaped by these words, not by a vague optimism outside the passage.