Passage
Let the house of Aaron now say, that his mercy endureth for ever.
Let the house of Aaron now say, that his mercy endureth for ever.
Psalms 118:1 O give thanks unto the LORD; for he is good: because his mercy endureth for ever.
Psalms 118:2 Let Israel now say, that his mercy endureth for ever.
Psalms 118:3 Let the house of Aaron now say, that his mercy endureth for ever.
Psalms 118:4 Let them now that fear the LORD say, that his mercy endureth for ever.
Psalms 118:5 I called upon the LORD in distress: the LORD answered me, and set me in a large place.
The verse centers on "mercy", "house", "aaron", "endureth", and "ever". It is saying that the verse draws attention to "mercy" and "house", so its meaning should be read from those terms before moving to application.
The nearby context moves from verse 2's "Let Israel now say that his mercy..." into verse 4's "Let them now that fear the LORD...", so "mercy" and "house" 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 "house" with trust shaped by these words, not by a vague optimism outside the passage.