Teaching, Keeping and Mediating Upon God’s Word
Joshua 1:8 (ESV) … “This Book of the Law shall not depart from your mouth, but you shall meditate on it day and night, so that you may be careful to do according to all that is written in it. For then you will make your way prosperous, and then you will have good success.”
The study of the law must be continuous; because, when it is omitted even for a short time, many errors slip in, and our memory grows rusty. Besides, when continuous study is neglected, many things become strange and difficult to practice. Therefore, God orders his servant to persist in the daily study of the law and never cease to pursue it as long as he lives. Whence it follows that those who show contempt for this study are blinded by their intolerable arrogance.
But why does God forbid the law to depart from his mouth rather than from his eyes? To take mouth by synecdoche for face is inane. I am certain that the word mouth applies primarily to a man who studies not only for himself but also for the benefit of a whole people whose government is his responsibility. So he is commanded to attend to the teaching of the law, in order that when he speaks about it, he may be able to do so with benefit to the people as a whole, as his responsibility requires.
Meanwhile, he is commanded by his own teachableness to give others an example of obedience. For there are many who have the law in their mouths in public, while at the same time they are the worst keepers of it. Joshua is therefore given both commands: to teach others and to conform his own conduct and himself wholly to the same standard.
 Haroutunian, J., & Smith, L. P. (1958). Calvin: Commentaries (p. 392). Philadelphia: Westminster Press.