Question: "What does Ephesians 6:12 mean when it says that we struggle against evil powers in high places?"
When you are trying to understand a passage, if you don't know the original language of the text well enough to examine it in that way, a good way to get a better feel for the range of meaning of the text is to compare several good translations.
King James Version: "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."
Young's Literal Translation: "because we have not the wrestling with blood and flesh, but with the principalities, with the authorities, with the world-rulers of the darkness of this age, with the spiritual things of the evil in the heavenly places."
New King James Version: "For we do not wrestle against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this age, against spiritual hosts of wickedness in the heavenly places."
Revised Standard Version: "For we are not contending against flesh and blood, but against the principalities, against the powers, against the world rulers of this present darkness, against the spiritual hosts of wickedness in the heavenly places."
English Standard Version: "For we do not wrestle against flesh and blood, but against the rulers, against the authorities, against the cosmic powers over this present darkness, against the spiritual forces of evil in the heavenly places."
Taking the best elements of the above, I would say the best way to translate this text would be:
"For we wrestle not against flesh and blood, but against principalities, against powers, against the cosmic-rulers (κοσμοκρατορας, or "world-rulers") of the darkness of this age, against spiritual wickedness in heavenly places."
This is speaking about the demons, who war against us, and who are the powers behind the evil of this age, and who reside in the aerial realm.
In Ephesians 2:2, St. Paul spoke of "the prince of the power of the air, the spirit who now works in the sons of disobedience". Commenting on this, St. John Chrysostom says:
"Here again he means, that Satan occupies the space under Heaven, and that the incorporeal powers are spirits of the air, under his operation. For that his kingdom is of this age, i.e., will cease with the present age, hear what he says at the end of the Epistle; “Our wrestling is not against flesh and blood, but against the principalities, against powers, against the world rulers of this darkness;” (Eph. 6:12) where, lest when you hear of world-rulers yo`u should therefore say that the Devil is uncreated, he elsewhere (Gal. 1:4) calls a perverse time, “an evil world,” not of the creatures. For he seems to me, having had dominion beneath the sky, not to have fallen from his dominion, even after his transgression" (Homily 4 on Ephesians).
One of the most important books on the spiritual life that every Orthodox Christian should read, and re-read, is "The Arena: Guidelines for Spiritual and Monastic Life," by St. Ignatius (Brianchaninov), which was recently re-published by Holy Trinity Publications in a revised translation. St. Ignatius discusses the meaning of this passage in chapter 43 of the Arena, and then talks about how we should wage the war against what this verse speaks of in chapter 44, and 45. I would recommend the entire book, but these chapters deal with this passage in great detail.
