5 Therefore does he warn and say, “We wrestle not against flesh and blood, but against principalities and powers.” Any one might suppose this meant against the kings of the earth, against the powers of this world. How so? Are they not flesh and blood? And once for all it is said, “not against flesh and blood.” Turn your attention from all men. What enemies then remain? “Against principalities and powers of spiritual wickedness, the rulers of the world.” It might seem as though he gave the devil and his angels more than they have.
It is so, he has called them the “rulers of the world.” But to prevent misunderstanding, he explains what this world is, of which they are the rulers. “The rulers of the world, of this darkness.” What is, “of the world, of this darkness?” The world is full of those who love it, and of unbelievers, over whom he is ruler. This the Apostle calls darkness. This darkness the devil and his angels are the rulers of. This is not the natural, and unchangeable darkness: this darkness changes, and becomes light; it believes, and by believing is enlightened.
When this takes place in it, it will hear the words, “For you were sometimes darkness, but now are you light in the Lord.” For when you were darkness, you were not in the Lord: again, when you are light, you are light not in yourselves, but in the Lord. “For what have you which you have not received?” Inasmuch then as they are invisible enemies, by invisible means must they be subdued. A visible enemy indeed you may overcome by blows; your invisible enemy you conquer by belief. A man is a visible enemy; to strike a blow is visible also. The devil is an invisible enemy; to believe is invisible also. Against invisible enemies then there is an invisible fight.
Source: Sermons on Selected Lessons of the New Testament (New Advent)