Something that has struck me again and again, is that without the devil - that is without the concept of autonomous purposive evil operating in the human world - Christianity is incoherent.
Of course, philosophical ingenuity can at least seem to make Christianity cohere without the devil - and indeed that is mainstream.
But there are three significant problems with the mainstream Christianity-minus-the-devil:
1. It probably doesn't not really work, even at a philosophical level.
2. It is anti-Scriptural, and flies in the face of most of Christian history.
3. The arguments make no sense for plain simple people who are not philosophers.
In other words, subtracting the devil makes Christianity nonsense for the very people who might make the best Christians.
Focusing on the third point: without the devil, at the level of plain simple analysis, when evil things happen they are explained as being done by God. Which means - in this non-intellectual way - that because God is Good, evil things are actually good. In other words, think of the most evil thing you can, and (without the devil) this was done by God for good reasons (which are no doubt obscure).
The above is not a parody, but is what plain people are being asked to accept from a Christianity minus the devil. No wonder the problem of evil (or the problem of pain) is responsible for so many people leaving Christianity.
Without the devil (and to the plain person), Christianity becomes merely a religion of submission to the incomprehensible will of God - meliorated by the promise that Christ's sacrifice and resurrection will make things right in the next life.
I am aware that the idea of the devil is now regarded as ridiculous or itself a source of evil; and that it can in practice become an excuse for evil ('the devil made me do it' etc).
But the concept is just essential to Christianity; without the devil we cannot make Christian sense of Christianity in a fashion which is comprehensible.
The reality of the devil is therefore non-negotiable for Christians; and - since the earth is full of evil - the use of the devil in explanations will not be exception but frequent, everyday.
Naturally, there are other sources of evil and reasons for bad things: especially original sin (the fault of humans, including ourselves), and random events.
Specifying a particular evil as the fault of the devil is therefore a conjecture, and may be wrong.
But it is a possibility that ought to be considered: and not as a last resort but as a normal everyday thing for Christians.
That is, indeed, how the devil has been regarded by all truly devout historical Christians of whom I am aware.