John Calvin Commentary Deuteronomy 31:19

"Now therefore write ye this song for you, and teach thou it the children of Israel: put it in their mouths, that this song may be a witness for me against the children of Israel." — Deuteronomy 31:19 (ASV)

Now, therefore, write you this song. It seems absurd that a useless remedy should be applied to an incurable disease. Why does God not rather correct their wickedness, and by His Spirit mold their hearts to obedience, than pour forth words in vain into their deaf ears? Thus proud and profane men mock at this mode of dealing with them, as if God, throwing away His labor, were deluding unhappy men.

We must bear in mind, however, that the preaching of the word, although it is a savor of death to those who perish, is still a sacrifice of sweet savor to God. Nor is it to be considered thrown away and ineffectual when it convicts the ungodly more and more, and renders them altogether inexcusable. And God expressly declares that this would be the use of the song as “a witness” against those from whose mouth it should proceed.

To some, indeed, it was profitable for salvation; for, when subdued by chastisement, they finally learned from it that their iniquities were the source and cause of all their evils. For, however God may redouble the blows of His scourges, unbelievers, who are without instruction, reap no advantage from them. Thus, this song was the means of assisting the elect to seek after repentance when they were smitten by the hand of God.

Still, although the word of God should do nothing more than condemn its hearers to death, yet it would be enough that it was a sweet savor to Himself. It seems by no means consistent with our reason that God should have given this command to Isaiah:

Go, and tell this people, Hear you indeed, but understand not; and see you indeed, but perceive not. Make the heart of this people fat, and make their ears heavy, and shut their eyes, lest they see with their eyes, and hear with their ears, and understand with their heart, and convert, and be healed (Isaiah 6:9–10).

But, with respect to the secret judgments of God, by which all our senses must be overwhelmed, let sober-mindedness be our wisdom.