(Read Joshua 1:5-9)
Joshua is to make the law of God his rule. He is charged to meditate therein day and night, that he might understand it. Whatever affairs of this world we have to mind, we must not neglect the one thing needful. All his orders to the people, and his judgments, must be according to the law of God. Joshua must himself be under command; no man’s dignity or dominion sets him above the law of God. He is to encourage himself with the promise and presence of God. Let not the sense of thine own infirmities dishearten thee; God is all-sufficient. I have commanded, called, and commissioned thee to do it, and will be sure to bear thee out in it. When we are in the way of duty, we have reason to be strong and very bold. Our Lord Jesus, as Joshua here, was borne up under his sufferings by a regard to the will of God, and the commandment from his Father.
For a more detailed summary select the link below.
Joshua 1 Bible Commentary – Matthew Henry (concise) (christianity.com)
Joshua 1:9
9 Haven’t I commanded you? Be strong and courageous. Don’t be afraid. Don’t be dismayed, for Yahweh your God is with you wherever you go.”