John Calvin Commentary Psalms 149:4

"For Jehovah taketh pleasure in his people: He will beautify the meek with salvation." — Psalms 149:4 (ASV)

For God hath taken pleasure in his people. We have spoken elsewhere of the verb רצה (ratsah); here it means free favor, the Psalmist saying that it was entirely of his good pleasure that God had chosen this people to himself.

From this source flows what is added in the second clause: that God would give a new glory of deliverance to the afflicted. In Hebrew, ענוים (anavim) means poor and afflicted ones, but the term was later applied to merciful persons, as bodily afflictions have a tendency to subdue pride, while abundance begets cruelty.

The Psalmist accordingly mitigates the sadness of present evils by administering seasonable consolation, so that God’s people, when oppressed by troubles, might look forward with hope to the glorious deliverance that was yet unseen. The sum of the passage is this: that God, who had fixed his love upon his chosen people, could not possibly abandon them to such miseries as they now suffered.