“ Be strong, and of good courage, for you will cause this people to inherit the land which I swore to their fathers to give them.”
Joshua was to be ‘strong', the word often indicates strength of hand. But his hand was to be strong because his spirit was strong. ‘Of good courage.' This word also indicates being strong, and especially strong in spirit. Thus ‘be doubly strong'. Strong in action, strong in heart, strong in spirit. (Compare Deuteronomy 31:7).
“For you will cause this people to inherit the land which I swore to their fathers to give them.” Notice the word ‘inherit'. It links closely with the word covenant. The land was to be theirs because YHWH had covenanted it to them by an oath. Because of this covenant it was theirs by right as a result of God's gracious covenant love. Thus their possession of it was inevitable. Compare Deuteronomy 31:7. The same idea is applied in the New Testament to our calling in Christ. That too we ‘inherit' because chosen and endowed by Him (Ephesians 1:11; Colossians 1:12; 1 Peter 1:4).
In these two verses YHWH brings to the mind of Joshua words of Moses spoken earlier as recorded in Deuteronomy. As he lay there in his dream they echoed and re-echoed in his mind. This is also true in the following two verses.