Edited from Chapter 13, the final one, of Unancestral Voice by Owen Barfield (1965).
By the Adversaries, Barfield means Lucifer and Ahriman, who are conceptualised as as actual but distinct aspects of 'Satan' - they are the adversaries of Christ's plan for the development of Men to the fully-divine form of consciousness. This idea comes from Rudolf Steiner, but is too complex to explain here.
In what follows, Barfield makes a subtle distinction concerning evil that I consider to be not just misleading, but false - however (in the parts I have emphasised in bold text) he also makes a deep, vital, important and neglected point. Square brackets indicate editorial additions.
"[The Adversaries seek, not transformation but either continuity or substitution.
"We have spoken little as yet of good and evil. If you are wise you will not think of the adversaries as evil.
"Not they, but what they do - if man fails to prevent it - is evil.
"They are not evil, because it is only in the activity of preventing them that Man achieves freedom: the freedom without his transformation [i.e. resurrection to eternal life] would not be self-transformation."
Now, I regard the best definition of evil to be that which opposes the divine destiny for Man; and in that respect, the adversaries certainly are evil; since opposition to God and to Jesus Christ is precisely what they do...
I think what Barfield means here is that the adversaries just are the way they are; and God has made this world such that Satan's activities are a part of it: in other words, opposition is allowed.
That is just a fact of mortal incarnate life. God has his reasons; which we personally may or may not understand - and which nobody understands in detail.
However, Barfield wants to put the focus on a place we seldom allow; which is that the major evil in this world comes from Man's failure to oppose, to prevent, that which the Adversaries want to do.
In a sense demonic evil is just a fact of life, about which we personally can do nothing. But demonic evil nearly-always works by Men's implementation. And another fact is that Men have freedom (agency) either to implement or to oppose evil.
Assuming that we are here to experience and learn from life - this is one vital thing we are here to learn: we are here to learn to 'prevent' evil; that is not to implement it, to work against it.
We should ideally never allow ourselves to think along the lines of 'the devil made me do it'. And when we do think like this, it must be repented.
What Barfield is saying here is that we need to learn to take responsibility for the evil that is done - and to take responsibility for 'preventing' evil as best we may; because that is to be free; and to be free, to embrace our being as a moral agent, is one of the essential things that God wants from us.