No man shall be able to stand before you all the days of your life. Just as I was with Moses, so I will be with you. I will not leave you or forsake you. Be strong and courageous, for you shall cause this people to inherit the land that I swore to their fathers to give them. Only be strong and very courageous, being careful to do according to all the law that Moses my servant commanded you. Do not turn from it to the right hand or to the left, that you may have good success wherever you go. 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. Have I not commanded you "Be strong and courageous. Do not be frightened, and do not be dismayed, for the Lord your God is with you wherever you go.
Alan Redpath's comments on this passage from Victorious Christian Living: Studies in the Book of Joshua:
Most of us, God forgive us, are too big for God to use. We are too full of our own schemes and of our own way of doing things. God has to humble us and break us and empty us. So low, indeed, must God make us that we need every word of encouragement from heaven to enable us to take on the job and dare to go forward in the will of God. The world speaks about the survival of the ﬁttest, but God gives power to the faint and He gives might to those who have no strength. He perfects His strength in weakness; He uses the things that are not to bring to nought the things that are.
Let me not be too big for God to use me; I'd rather be weak and have his encouragement "Be strong and courageous!"