For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places.
Before giving us the armor, God continues to stress the importance of why we need this armor. Our battle is not physical - thus, we cannot rely on physical resources (example: Psalm 20:7, "Some trust in chariots, and some in horses: but we will remember the name of the Lord our God."). To fight the spiritual battle warring, we must arm ourselves with spiritual resources.
We cannot be ignorant against whom we are fighting. The Bible helps us to understand the enemy: "Be sober, be vigilant; because your adversary the devil, as a roaring lion, walketh about, seeking whom he may devour: Whom resist stedfast in the faith, knowing that the same afflictions are accomplished in your brethren that are in the world." (1 Peter 5:8-9)
The devil is as a roaring lion.
He walks about.
He seeks to devour.
But, he can be resisted.
Wherefore take unto you the whole armour of God, that ye may be able to withstand in the evil day, and having done all, to stand.
Just as we must "put on" the armor, we must also take the armor. These words give the implication that it is our choice whether or not we will receive the armor that God has provided. It is there. It is available. But do we take it?
After warning us about the devil, the challenge again to pick up the armor of God and use it is given - very similar to verse 11. We do not have to be defeated by the wiles of the devil. Instead, we can withstand (stand against, oppose) and stand (to stand).