A Public Profession of Faith
Before their first battle, the Lord commanded the Israelites to renew their commitment to Him through circumcision. This was a sign of God's covenant to give them the Promised Land and make them into a great nation, yet their father's had broken this covenant and failed to circumcise them. So, now that they are in the land and God will soon fulfill His covenant with them, they must do their part in obedience, faith, and allegiance to God. It was an outward sign of their inward faith just as baptism is for us today. It is our public profession and a picture of our sins being buried with Him and washed away as we are raised with Him anew.