Albert Barnes Commentary Isaiah 30:17

"One thousand [shall flee] at the threat of one; at the threat of five shall ye flee: till ye be left as a beacon upon the top of a mountain, and as an ensign on a hill." — Isaiah 30:17 (ASV)

One thousand... The meaning of this is that you will be easily alarmed and overcome by those who are inferior in numbers and strength. The number ‘one thousand’ is used for a large, indefinite number, probably meaning all.

At the rebuke of one. The number one here is used to denote a very small number; a number that, in the ordinary course of warfare, is entirely disproportionate to those who would be defeated. There is probably a reference here to the prediction in Deuteronomy 32:30:

How should one chase a thousand,
And two put ten thousand to flight,
Except their Rock had sold them,
And Yahweh had shut them up?

At the rebuke of five—of a very small number.

Until you are left as a beacon upon the top of a mountain. The word translated ‘beacon’ (Hebrew: תרן, toren; Greek: ἱστὸς, histos—meaning “a mast”) properly denotes the mast of a ship (Isaiah 33:23; Ezekiel 27:5), then anything resembling a mast, a flagstaff, or a beacon of any kind.

It may refer to a staff or mast erected on a promontory to warn sailors or to be a landmark—as it is not improbable that the masts of ships would be used for that purpose. Alternatively, it may refer to a flagstaff, erected in a conspicuous place, to which the nation could rally in time of war. On the seacoasts of America, such beacons are often erected. Those which I have seen consist of a pole erected on an eminence or rising ground, with a cask or barrel painted white on the top.

The idea seems to be that of a long pole erected for any purpose, standing alone, stripped of its leaves and branches, and without ornament. So would be the few, solitary, and scattered Jews when driven before their enemies.

And as an ensign on a hill (see note on Isaiah 5:26). The idea is that those who would escape would be few in number and would stand alone, like a beacon in view of all the nations, to admonish them of the justice of God and the truth of His threatening—like an ensign floating on a hill that can be seen from afar. What a striking description this is of the condition of the Jews in our times, and indeed in all ages since their dispersion! Their strength, influence, and power as a people are gone. They stand as beacons to warn the nations of the evils of a lack of confidence in God, and of His justice.