Specifically, images of "warfare," with light being opposed to darkness, makes us think that Good and Evil are an ontological pair, eternally pitted against each other. To be sure, there are some religious cosmologies that embrace that dualism, but Christianity isn't one of them.
And yet, Christians see themselves as battling dark forces, collectively described as "the satan," the Adversary.
So how do we avoid drifting into dualism in light of that struggle?
In Reviving Old Scratch I borrow from the work of scholars like Louis Martyn, his commentary on Galatians in particular, to argue that the reason we experience spiritual warfare isn't because of dualism but because God's invasion of the world. Scholars call this framework apocalyptic. The world is held in captivity by dark cosmic forces--the Devil, Sin and Death--and God invades the cosmos to liberate humanity. Yes, this is Christus Victor atonement.
The key, though, for the concern about dualism, is that we experience warfare not because of ontology but because of God's apocalyptic invasion of "the present evil age."