John Calvin Commentary Exodus 17:14

"And Jehovah said unto Moses, Write this for a memorial in a book, and rehearse it in the ears of Joshua: that I will utterly blot out the remembrance of Amalek from under heaven." — Exodus 17:14 (ASV)

And the Lord said to Moses. By this command God made it known that He had performed a work which not only ought to be celebrated by word¹⁹² of mouth, but which also was deserving of eternal glory with future generations; therefore, He commanded it to be written in a book, so that its memory might never perish.

The dispute among commentators regarding this book seems to me to be superfluous; for God simply wills that the memorial of this event should exist in all ages. This was accomplished through the account of Moses, as he handed down in writing—even to the end of the world—the praise of this favor, along with the perpetual and immortal doctrine of the Law.

Yet God not only wished for the memorable event of this battle to be written down, but also for Joshua to be reminded of it, so that he would not lose heart amidst the many difficulties that awaited him. For nothing could better support him with invincible firmness than the recollection of this history, from which he could be assured that the people would always be victorious under the auspices of God.

But although this promise was not immediately fulfilled—the Amalekites being totally destroyed by Saul a long time afterwards—it was nevertheless a great encouragement to Joshua and the people to know that Amalek, who had first made war upon them, was already condemned by the divine decree and could not escape the destruction to which he was doomed.

¹⁹² “Par la bouche de ceux qui vivoyent alors;” by the mouth of those who were alive then. — Fr..