What do you think about The Devil?
It strikes me that this is a good discriminatory question to ask self-identified Christians; to see whether they are real Christians or something else.
This can work both ways. It can reveal that a self-styled Christian is instead a spiritual New Ager/ perennial philosopher/ Western-Eastern-religioner; or perhaps a 'liberal' (or fake) Christian (actually a Leftist fifth columnist).
It may reveal that somebody who sounds like 'one of the above' is actually a real Christian - when pushed (and this question does push us, at least nowadays). There is sometimes more to such people than first meets the eye - and this can be seen in their expressed attitude to the devil, and to the demonic agenda and activity.
Or it may reveal that strangest of modern types: one who believes in the devil - perhaps practices a demonic religion - but does not believe in God. I think such people are pretending to themselves that their 'Satanic' belief is ironic, playful, political, artistic, magical...
But it is actually the real thing, whether they acknowledge it or not; and it has demonstrable spiritual (and indeed behavioural) consequences.