C.H. Spurgeons's The Treasury of David
Verse 6. Cast forth lightning, and scatter them. The Eternal can hurl his lightnings wheresoever he pleases, and effect his purpose instantaneously. The artillery of heaven soon puts the enemy to flight: a single bolt sets the armies running hither and thither in utter rout.
Shoot out thine arrows, and destroy them. Jehovah never misses the mark; his arrows are fatal to his foes when he goes forth to war. It was no common faith which led the poet king to expect the Lord to use his thunderbolts on behalf of a single member of that race which he had just now described as "like to vanity." A believer in God may without presumption expect the Almighty Lord to use on his behalf all the stores of his wisdom and power: even the terrible forces of tempest shall be marshalled to the fight, for the defence of the Lord's chosen. When we have once mastered the greater difficulty of the Lord's taking any interest in us, it is but a small thing that we should expect him to exert his great power on our behalf. This is far from being the only time in which this believing warrior had thus prayed: Psalms 18:1-50 is specially like the present; the good man was not abashed at his former boldness, but here repeats himself without fear.
EXPLANATORY NOTES AND QUAINT SAYINGS
Verse 6. Cast forth lightning. The Hebrew here is, "Lighten lightning"; that is, Send forth lightning. The word is used as a verb nowhere else. --Albert Barnes.