We have already gotten God s approval, He sent Christ to be righteousness for us. No trick or scheme of Satan can take away that righteousness unless we let down our breastplate and bare our heart for him to take. By holding the sacrifice of Christ against our heart as a shield, we cannot be lost by attack to the heart. Here Paul is telling us that we must be prepared to tell the Gospel, to run and spread the Good News. To do so, God has covered our feet making us able to travel long distances without injury that would slow us down.
Shoes are important armor.
If an army can t stand from wounds to its feet, it can scarcely do battle. Satan would love for us to have wounds to our spiritual feet. The last thing he wants is to have us running around as fast as we can spreading the Word. He ll attack our feet by telling us the work is too hard, that there is no one that will listen, that we will be branded crazy if we try to spread the Good News. He s just trying to steal your shoes.