Summary: Since God is our power, or strength, our fortress, our high tower, we can stand-and that is our role in the fight. God takes care of the rest. For us, standing is sometimes difficult. But standing is enough.
10 Finally, be strong in the Lord and in his mighty power. 11 Put on the full armor of God, so that you can take your stand against the devil’s schemes. 12 For our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms. 13 Therefore put on the full armor of God, so that when the day of evil comes, you may be able to stand your ground, and after you have done everything, to stand.
C.S. Lewis refers to this passage in a discussion between two characters in one of his space novels and proffers his own translation and application of verse twelve thus:
When the Bible used that very expression about fighting with principalities and powers and depraved hypersomatic beings at great heights (our translation is very misleading at that point, by the way) it meant that quite ordinary people were to do the fighting." –Paralandra
We are fighting in a war. In this way Paul returns to the theme of being in Christ-we are to be strong in Him in the battle against powers of darkness. Think of the many hero stories you’ve heard. Frodo finds out he has had a ring-The Ring of Power. Spiderman gets a bite from an energized spider. Superman, raised by humans, finds he has unearthly power because he was actually born on another planet, and his presence here gives him considerable strength. Paul is saying it’s like that with us. We have the Power of the Holy Spirit. We’ve been bitten by the energized arachnid of God. We have a new birth from heaven (a home which, unlike Krypton, will never be destroyed) that gives us considerable strength, and we have to learn how to use it. After all, with great power comes great responsibility.
Paul teaches we are in a spiritual battle. The problem is we view many things as merely physical. We face a decision concerning a relationship and think of it in emotional terms, not recognizing the spiritual impact of that decision. We consider how to use the resources God has placed in our hands and don’t realize we can become powerful weapons in God’s hands to fight poverty and illness and darkness and despair. Our senses tell us we are physical beings, with every thought a blip of electrochemical energies. The Bible teaches there is more to us-a superhero backstory we all need to learn. AS Pierre Teilhard de Chardin said, "We are not human beings having a spiritual experience; we are spiritual beings having a human experience." It’s like Elisha, surrounded in the early morning by an army. His servant came to him, deeply disturbed. Elisha prayed a simple prayer, ““Open his eyes, Lord, so that he may see.” Then the Lord opened the servant’s eyes, and he looked and saw the hills full of horses and chariots of fire all around Elisha.” (2 Kings 6:17) Like Elisha’s servant, our eyes are often closed to the spiritual realities around us.