For they shall soon be cut down like the grass - As the grass in the field is cut down by the mower; that is, however prosperous they may seem to be now, they are like the grass in the meadow which is so green and luxuriant, but which is soon to fall under the scythe of the mower. Their prosperity is only temporary, for they will soon pass away. The idea in the word rendered “soon” - מהרה mehêrâh - is that of “haste” or “speed:” Psalms 147:15; Numbers 16:46; Deuteronomy 11:17. The thought is not that it will be done immediately, but that “when” it occurs it will be a quick and rapid operation - as the grass falls rapidly before the mower.
And wither as the green herb - When it is cut down. That is, not as the dry and stinted shrub that grows in the desert of sand, but like the herb that grows in a garden, or in a marsh, or by the river, that is full of juices, and that needs abundant water to sustain it - like the flag or rush (compare Job 8:11) - and that withers almost instantly when it is cut down. The rapidity with which things “wilt” is in proportion to the rapidity of their growth, so the prosperity of a sinner is suddenly blasted, and he passes away. Compare Psalms 90:5-6.