Charles Ellicott Commentary Psalms 115:9

"O Israel, trust thou in Jehovah: He is their help and their shield." — Psalms 115:9 (ASV)

O Israel. —There is masterful art in this sudden change of address. It is like the pointed application of some general truth in a sermon. It is possible that in the liturgical use a change in the music was made here, the Levites and choir turning to the people with a loud burst of song.

He is their help and their shield. —The original form of this motto of trust appears in Psalms 33:20. Here the change of person suggests some musical arrangement. Apparently one part of the choir, or, it may be, one officiating priest, addressed successively the whole congregation with the charge, “trust in Jehovah,” and each time the full choir took up the refrain, “He is their helper and shield,” repeating to the priest the ground on which he urged confidence and loyalty. Then in Psalms 115:12-13 congregation and choir join, changing to the first person.