John Calvin Commentary Isaiah 38:21

"Now Isaiah had said, Let them take a cake of figs, and lay it for a plaster upon the boil, and he shall recover." — Isaiah 38:21 (ASV)

And Isaiah said Isaiah now relates what was the remedy which he prescribed to Hezekiah. Some think that it was not a remedy, because figs are dangerous and hurtful to boils; but that the pious king was warned and clearly taught by this sign that the cure came only from the favor of God alone.

As the bow in the sky, by which God was pleased to testify that mankind would never be destroyed by a flood (Genesis 9:13), appears to denote what is absolutely contrary to this (for it appears when very thick clouds are gathering and are ready to fall, as if they would deluge the whole world); so they think that a plaster, which was not at all suited for curing the disease, was purposely applied by the Prophet to testify openly that God cured Hezekiah without medicines.

But since figs are employed even by our own physicians for maturing a pustule, it is possible that the Lord, who had given a promise, also gave a medicine, as we see done on many other occasions. For although the Lord does not need secondary means, as they are called, yet he makes use of them whenever he sees fit. And the value of the promise is not lessened by this medicine, which without the word would have been vain and useless, because he had received another supernatural sign, by which he had plainly learned that he had received from God alone that life of which he despaired.