The Israelites could not hope to overcome the fortified cities in their own strength. And you and I cannot hope to overcome the sin that entangles us in our own strength either. Yet, just as the Israelites could not sit and do nothing and wait for God to deliver the cities to them, we also must do what we know to do, follow the Holy Spirit's leadership, and trust that the battle is already won because of Christ's finished work.
As Pastor J.D. said, "Christ's words on the cross were not, 'It's almost finished; you take it from here.' He said, "IT IS FINISHED."
Brothers and sisters, it is not just our salvation which is purchased and settled on the cross, but our sanctification as well. Sure, we still have to march in and claim the territory, but the battle was won 2000 years ago. Whether your specific battle is sexual sin, addiction, pride, self-righteousness, racism, or vanity, Jesus defeated the enemy that wants to hold you captive.
Just as the Israelites were to leave no survivors as they took possession of the land God had given, you and I must be merciless as we trust the Holy Spirit to eliminate every stronghold. We must be merciless in examining and putting an end to sin in our own life. But we must also trust the work that God is doing in the lives of others.
Since God is the one who defeats sin, our role is not to condemn those who haven't yet discovered victory. God is working in their life and His timetable is between Him and His child. In Roman's 14:4, we are told, "Who are you to judge someone else’s servant? To their own master, servants stand or fall. And they will stand, for the Lord is able to make them stand."