Joshua had been one of two out of the original twelve spies who had entered into the promised land to seek out evidence that the land was as God said it would be. As the land was discovered to be as it should be, the people still refused to enter in and as a result, the Lord kept that generation from entering; with the exception of Joshua and Caleb.
During the 40 years of wandering, Moses, Joshua's mentor, as well as the rest of his generation died.
With a whole new generation of people to be led, a land promised by God still to be occupied, and the mantle of command placed on his shoulders, Joshua would need encouragement to proceed. God's promise to never leave or forsake Joshua and his people.
We as the people of God have as much need for this promise as the people of Israel did. We still have to live out the common life in the character of Christ in our day to day ministrations to our families, work-mates, and associates. The fact is, the truest religious life is that in which a man follows the ordinary calling of life in the spirit of a Christian. Now, are you doing so? If so, you are as much ministering before God in measuring out stocks in the market, or stocking shelves in the store, as Joshua was in slaying Hivites, Jebusites, and Hittites! You are as much serving God in looking after your own children and training them up in God’s fear, and minding the house and making your household a church for God, as you would be if you had been called to lead an army to battle for the Lord of hosts! And you may take this promise for yourself, for the path of duty is the path where this promise is to be enjoyed. “I will not fail you, nor forsake you.”