Faith McDonnell thinks that the war (yet to be declared by the U.S. government) against ISIS is spiritual in nature and that Christians have a duty to wage war against such enemies:
First and foremost, the battle against ISIS is spiritual warfare. If the Enemy ever used any proxies for his mission to “kill, steal, and destroy,” it is the Islamic State. Though some remain in denial about the evil we face, and pray just “for peace,” the Biblical mandate for Christians (not governments, not militaries ) is to battle — 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.” (Eph 6: 12)
When you read Calvin, who is no pope, on Ephesians 6, you gain the impression that Paul may have had his sights set on a foe bigger than Rome’s armies:
To impress them still more deeply with their danger, he points out the nature of the enemy, which he illustrates by a comparative statement, Not against flesh and blood. The meaning is, that our difficulties are far greater than if we had to fight with men. There we resist human strength, sword is opposed to sword, man contends with man, force is met by force, and skill by skill; but here the case is widely different. All amounts to this, that our enemies are such as no human power can withstand. By flesh and blood the apostle denotes men, who are so denominated in order to contrast them with spiritual assailants. This is no bodily struggle.
Let us remember this when the injurious treatment of others provokes us to revenge. Our natural disposition would lead us to direct all our exertions against the men themselves; but this foolish desire will be restrained by the consideration that the men who annoy us are nothing more than darts thrown by the hand of Satan. While we are employed in destroying those darts, we lay ourselves open to be wounded on all sides. To wrestle with flesh and blood will not only be useless, but highly pernicious. We must go straight to the enemy, who attacks and wounds us from his concealment, — who slays before he appears.
In fact, Calvin suggests that if we fight enemies like ISIS as if they are spiritual opponents, we lose sight of our genuine spiritual adversaries. And when that happens, our spiritual enemies defeat us.
It’s a topsy-turvy world out there in Christ’s spiritual kingdom.