John Calvin Commentary Isaiah 37:27

"Therefore their inhabitants were of small power, they were dismayed and confounded; they were as the grass of the field, and as the green herb, as the grass on the housetops, and as a field [of grain] before it is grown up." — Isaiah 37:27 (ASV)

For their inhabitants were maimed. Here the Prophet expresses more fully what he previously touched upon briefly: that we should not judge the condition of the Church by the stability of this world. For although fortified cities are taken, and the strongest men lose courage and fall into the hands of their enemies, yet the Church shall stand and flourish, because it does not rest on its own strength, and its foundation is not on earth but in heaven.

There is thus an implied contrast between fortified cities, which alarmed and terror-stricken inhabitants are unable to defend, and the Church of God, which rests on His grace alone. Therefore, it resists every attack, so that it never fails, for the Church refers everything to God alone, from whom she receives the beginning of life, uninterrupted strength, perseverance, every part of salvation, and every blessing.

Thus we learn that all fortresses are of no use if the hand of the Lord does not assist. All human strength will be broken and decay if it is not supported by His power; castles, bulwarks, and the most powerful armies will be of no use without Him. This is expressed more fully by the following metaphors.

Like the grass of the field and the green herb. It is important that believers should be led to admire the wonderful grace God shows toward them, so that they do not envy the earthly wealth possessed by irreligious people. Although their power may be dazzling and magnificent, He shows that they are like the green herb and the grass, which indeed flourish for a time but quickly wither. He dwarfs them still more by another metaphor He adds.

The grass of the housetops. It is indeed lofty and seen by all, but the more elevated its position, the nearer it is to the heat, and it withers more quickly and is of no use whatever. And it is also said in the psalm that:

they who pass by do not bless it (Psalms 129:8).

Though the enemies of the Church are high and, as it were, exalted to heaven, though they flourish and have abundant wealth, yet they quickly fall. Therefore, just as the grain that lies on the ground at our feet is more valuable than the unfruitful herb that grows on the housetops, so the Lord shows that the lowly condition of His servants is far more desirable than that of those who, leaning on their own strength, vaunt themselves above others and boast against God Himself.

Which withereth before it is ripe. Some think that this is a fourth comparison, but I think that the Prophet added it for the sake of exposition, as if he had said that grass of this kind withers before it develops a stalk—that is, before it is fully ripe. This is just as it is said in the psalm: Before it is grown up, it withereth (Psalms 129:6).