John Calvin Commentary Deuteronomy 31:23

"And he gave Joshua the son of Nun a charge, and said, Be strong and of good courage; for thou shalt bring the children of Israel into the land which I sware unto them: and I will be with thee." — Deuteronomy 31:23 (ASV)

And he gave Joshua the son of Nun a charge. The more difficult Joshua's task was, the more necessary it was for him to be encouraged to exert himself and to beware of failure.

For this reason, his charge was repeated, although in his person, all the others were also confirmed at the same time.

Moses based this charge on God's promise, which has been mentioned so often, and stated that Joshua had been chosen to complete the work of deliverance already begun.

Indeed, it was scarcely believable that the disciple should not only be superior to his master, but also that a man of humble position should be elevated to the dignity from which the sovereign Prophet and God’s chief minister had been degraded, unless this was done by God's decree and ordinance.

At the same time, however, Moses made Joshua more confident about the outcome of his calling by promising him that God, who was the initiator of this expedition, would be with him. For He has the power to accomplish every work to which He has appointed any one of us.