This book of instruction must not depart from your mouth; you are to meditate on it day and night so that you may carefully observe everything written in it. For then you will prosper and succeed in whatever you do.
Joshua 1:8 (CSB)

Joshua is here assuming leadership from Moses. He is portrayed as the second Moses. Just as God affirmed Moses’ leadership through miracles, signs, and wonders, God affirms Joshua by performing similar miracles under his leadership. Moses parted the red sea (Ex. 14), Joshua parts the Jordan River (Josh. 3-4). Moses acted as a priest and ratified the covenant between God and the people with the tablets of stone and the blood of the sacrifice sprinkled on the people (Ex. 24). Joshua also acts as a priest and renews the covenant at Shechem before his death (Josh. 24).

The above verse stands as the first bookend of the book of Joshua, with God commanding Joshua not to depart from the instruction given by Moses. The book ends with Joshua commanding the people not to depart from following the Law. God is instructing Joshua and the Israelites to meditate on the Law and keep it so that they may not be like their fathers who served the gods beyond the river in Egypt or the Gods of the Canaanites in whose land they now dwell. The only way to please the Lord was to follow his ways. So, the Israelites could be an example to the neighboring nations of how to live life and have a right relationship with God. By doing this, they would be a kingdom of priests and a holy nation (Ex. 19:6).

The only way we can please God is by believing in his Son, Jesus Christ. Like the Israelites, we must also meditate on God’s word and keep it, work out our salvation with fear and trembling, and be a witness to this present generation that is hostile to God. In this way, we fulfill the command of being a royal priesthood, a holy nation, called to proclaim the excellencies of Him who has redeemed us out of darkness into his marvelous light (1 Pet. 2:9).

God’s instruction to Joshua about prospering and succeeding because of obedience is very similar to Moses’ teaching the Israelites about the blessings or curses that follow because of obedience or disobedience to the Law. The success or prosperity of Joshua is in the temporal sense of winning the conquest and leading the Israelites well, but for us, this prosperity is in the eternal sense of being in God’s kingdom forever. More than that, it is not the commands of Moses that we keep, but the commands of Christ, who is the perfect fulfillment and embodiment of the Law.