It is necessary for Christians to believe in the reality of supernatural purposive evil (as conceptualised, typically, in terms of Satan, demons, fallen angels etc.)
This is non-optional - however uncomfortable and embarrassing to the modern sensibility; because it is scriptural and the two millennium teaching of Holy Fathers and Saints.
It is also necessary in order to avoid serious misunderstanding.
People seem to talk as if supernatural purposive evil is either an unnecessary hypotheses or an hypothesis of last resort.
Both are errors.
The proper question is whether something is true, not whether an hypotheses is un-necessary - indeed, strictly, any hypotheses can be discarded, if you are prepared to accept the consequences.
Nor should it be an hypothesis of last resort - but it should be an hypothesis which is applied where and whenever it is applicable - whenever the proper explanation is that of supernatural purposive evil at work.
If, as most modern people have, we discard supernatural positive evil as a fact operative in the world - what are we left with?
Only two types of explanation for evil:
1. Selfish short-termism.
3. Well-intentioned error.
In other words we get the Leftist perception of evil as being due to stupid, ignorant, selfish, frightened, greedy people (ie. Right Wingers - such as Hitler) who therefore require more Leftist 'education'/ propaganda/ discipline; or else evil is excused as an accidental and unintentional result of well-meaning and nobly-motivated Leftists (such as Stalin and Mao).
Without the concept of supernatural purposive evil, evil is thus explained-away.
Since all are selfish and short-termist, discussion is merely a matter of unmasking this in others in order to distract from it in oneself.
Well-intentioned error is treated as a technocratic problem - the problem of evil is reduced to a five year plan to ensure than good intentions are effectively implemented.
In face of this pitifully flat, simplistic and impoverished vision of reality, we must acknowledge the active pursuit of evil for its own reasons, as the motivated destruction of Good.
Those who serve this evil intent (in varying degrees) may do so for selfish reasons of expediency, or through error, or deception - or may make the choice actively to destroy the Good.
(As have many modern intellectuals: artists who work to destroy Beauty; public relations people who work to destroy Truth; journalists and pressure groups who work to destroy Virtue - to be subversive and inverting of Good is mainstream modern intellectual life.)
The fact that such folk stand against the consensus of the ages and deny the reality or even the possibility of supernatural purposive evil is significant, indeed decisive.
Of course, as corrupt and imperfect creatures - we certainly will make errors in applying the hypothesis of supernatural purposive evil to any specific situation - as we make errors in applying science to any specific situation; and in a sense we can never be sure of anything.
Nonetheless, we must do our best; be aware of the possibility of error; and ready to correct it
And of course there are more concrete/ simple and more abstract/ complex ways of describing purposive evil - e.g. as personified devils or fields of influence.
But bear in mind abstraction is not all gain - it has a tendency to make things seem less real; also that abstraction is merely another kind of metaphor, but less obviously so than anthropomorphic metaphors.
But the point is that we should acknowledge the reality of supernatural purposive evil and be alert to the necessity of applying the hypothesis in order to understand the world accurately.
Not just 'should acknowledge' - we must.