John Calvin Commentary Joshua 1:8

"This book of the law shall not depart out of thy mouth, but thou shalt meditate thereon day and night, that thou mayest observe to do according to all that is written therein: for then thou shalt make thy way prosperous, and then thou shalt have good success." — Joshua 1:8 (ASV)

This book of the Law, etc. Assiduous meditation on the Law is also commanded. This is because whenever study is interrupted, even for a short time, many errors readily creep in, and the memory becomes rusted. Consequently, many who cease from continuous study engage in practical matters as if they were mere ignorant novices. God therefore commands His servant to make daily progress and never cease, during the whole course of his life, to advance in understanding the Law. Hence it follows that those who hold this study in disdain are blinded by intolerable arrogance.

But why does He forbid him to allow the Law to depart from his mouth rather than from his eyes? Some interpreters understand that the mouth is here used by synecdoche for face; but this interpretation is unconvincing. I have no doubt that the word used is particularly applicable to a person who was obligated to pursue this study, not only for himself individually, but for all the people under his rule. He is instructed, therefore, to pay attention to the teaching of the Law, so that, in accordance with the office entrusted to him, he may present what he has learned for the common benefit of the people. At the same time, he is ordered to make his own teachableness a pattern of obedience for others. For many, by talking and discussing, have the Law in their mouth but are very poor keepers of it. Both things, therefore, are commanded: that by teaching others, he should also make his own conduct and entire character conform to the same rule.

What follows in the second clause of the verse shows that everything ungodly people try to achieve while despising the word of God must ultimately fail. However prosperous the beginning may sometimes seem, the outcome will be disastrous. This is because prosperous results can be hoped for only from divine favor, which is justly withheld from rashly adopted plans and from all arrogance that is usually accompanied by contempt for God Himself. Therefore, so that their affairs may turn out as they wish, let believers secure the divine blessing both by diligence in learning and by faithfulness in obeying.

At the end of the verse, because the term used is ambiguous, as I have already observed, the sentence is repeated, or a second promise is added. The latter is the view I take. For it was most fitting that, after the promised success, Joshua should be reminded that people never act skillfully and properly except insofar as they allow themselves to be governed by the word of God. Accordingly, the prudence that believers learn from the word of God is opposed to the confidence of those who consider their own judgment sufficient to guide them correctly.