I will not fail thee, nor forsake thee. — Compare Genesis 28:15. And consider Hebrews 13:5 as a combination of the two Old Testament passages.
(6,7) Be strong and of a good courage... that thou mayest observe to do according to all the law. — This command to “be strong,” repeated again and again to Joshua, may perhaps be taken as reflecting light upon his natural character, which might not have led him to desire so prominent a position. But it may also be observed that courage was especially needed to carry out the conquest of Canaan in the way that was ordered by the law. For a discussion of this question and its difficulties, see Joshua 13