First, Joshua points out who they are: "You are a numerous people..." They lack assurance in their identity since they constantly thought of themselves as less than the Canaanites. Do I know who I am? My position in Christ is one of a child of God. I am vested with all the benefits of the kingdom by the grace of God. My identity is not based on anything I can do or merit. It is completely based on the work of Christ on the cross. He called me from darkness into light; from death unto life.
Secondly, Joshua reminds them of their resources: "... and have great power." In their complaint to Joshua, Ephraim and Manasseh did not fail to compare themselves with the Canaanites. The Canaanites' military might included chariots of iron. But God had granted great power to them because God fought for them. God has given me power to be his witness through the Holy Spirit who dwells in me. I must not fear the resources of the world and focus on the resources God has given me.
Thirdly, Joshua tells them that they must work and toil to enjoy their reward: "... you shall clear it ... and you shall drive out the Canaanites." God fought for them and gave them an inheritance, but they must continue to work and toil to expand their territory and enjoy it to the fullest. Christ has given me all I need. I need not work to get into the kingdom, but I must work to keep what has been given me. It is not a work to merit, but a work to maintain. This work is totally dependent on the grace and mercy of God, but work I must. No lazy bums in the kingdom!
Finally, Joshua points to the promised future: "For you shall drive out the Canaanites..." If they are willing to embrace their identity, realize the power they've been given, and work hard to expand their territory, God would grant them the victory despite their chariots of iron. My victory in Christ is assured if only I am willing to work out my salvation. I must persevere to the end to be saved. I must wage the battle of faith day by day to establish God's kingdom upon the earth. His promises are true and will become a reality if only I press on.
Father, thank you for your Son Jesus Christ whose work on the cross saves me. Thanks for the power of the Holy Spirit that fills me. May I work day by day to establish your kingdom. Rebuke laziness and complacency in me. Fill me with hope in your promises.