John Calvin 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)

A thousand, as one, shall flee at the rebuke of one. Because the Jews, on account of their vast numbers, relied on their forces, as men are accustomed to do when they possess any power, the Prophet therefore threatens that all the protection they have at home will be of no more use to them than foreign aid. The Lord will break and take away their courage, so that they will not be able to make use of their forces.

For of what use are arms and a vast multitude of men? Of what use are fortresses and bulwarks when men’s hearts fail and are dismayed? It is therefore impossible for us to be strong and powerful unless the Lord strengthens and upholds us by his Spirit.

This statement occurs frequently in the Law: that if they revolt from God, a vast number of them would be put to flight by a very small number of enemies. But there is this difference between the Law and the prophets: the prophets apply to a particular subject what Moses announced in general terms, as we have previously explained.

Here two observations must be made:

1. We have just as much courage as the Lord gives us, for we immediately lose heart if he does not support us by his power.
2. It is the result of God's righteous vengeance that we are terrified by men when he could not persuade us to fear him. When we have despised God’s word and warnings, we then fall down in terror at the words and threats of men.
3. God does not need extensive preparations to chastise us, for if he lifts up only a finger against us, we are undone. A small and feeble army will be sufficient to destroy us, even though we are well prepared and have great numbers on our side.

Next, the Prophet threatens that there will be no end to these calamities until they have been reduced to the last extremity, and until, amidst the frightful desolation of the earth, few tokens of God’s compassion are left.

As the mast of a ship on the top of a mountain. This can be explained in two ways. Some consider the metaphor to be taken from trees that have been cut down; for when a forest is cut down, lofty trees are left that can be used for building ships. But הר (hār), “a mountain,” probably also denotes a rock or promontory against which ships are dashed, to which they adhere, and on which a “mast,” the emblem of shipwreck, is afterwards seen.

As a banner on a hill. Another metaphor is now added, borrowed from trophies erected to commemorate the defeat of enemies. In short, the Prophet declares that they will be so few that all that remains will be an indication of very great ruin. It is as if he had said, “This great multitude that you now have dazzles your eyes, but there will be such ruin and decrease that you will no longer have the appearance of a people.”

We are thus reminded how humbly and modestly we should conduct ourselves, even if we have great wealth and numerous forces. For if our minds are puffed up, God will quickly beat down our pride and make us more feeble and cowardly than women and children, so that we will not be able to bear the sight of even a single enemy, and all our strength will melt away like snow.