He who for us is life itself descended here and endured our death and slew it by the abundance of his life.
Augustine’s revolutionary insight insists on the insubstantiality of evil. Evil has no life of its own, since it is the wilful distortion of the good, the prideful misshaping of the divinely-ordered creation. As Delbanco notes, the image of the knot enables Augustine “to speak of the complexity and intransigence of evil without granting it any essence of its own. A distortion of something whose essence precedes its disfigurement, evil is in the twistedness, not in the rope.” Thus does Augustine deliberately guard against the Manichean error of making evil the co-equal counterpart of good - indeed, a virtual provocateur of the good, arousing it to necessary warfare against evil.
The paradoxical Christian counter-claim is that, while evil has horrific power and devastating effect, it is ultimately to be regarded as emptiness and nothingness. It is something alien rather than essential to God’s good creation. It has no power to create, only to damage and destroy. Indeed, it exists only parasitically, leeching off the good, as the very name of C.S. Lewis’s demonic Screwtape makes evident: he is akin to a pernicious tapeworm feeding off his host.