Well, 'The Devil' is the antitheses to Jehovah. So you kinda have to at least believe in a creature or ideal in existence that would be necissary for the Devil to exist. I mean its popularly believed that the Devil comes from human minds, not from spiritual means. I dunno. But the likes of ancient Satanism existed before the days of Christianity, when Satan was first applied to The Devil, you know?
Anyway, what do you believe, exactly? Do you believe in any of the Devils buddies, aswell? Like, y'know. I don't know their names, there were 7 of them, sat on pillars or something.
If you believe in Christian mythology lucifer (the devil) was originally an angel but was cast down to hell by "God" for, I think, rebelling against gods rule of heaven. Then yes, you must believe In one to credit the other.
Here's the easy solution. Don't believe in either.
If you believe in the devil. Then that is a good thing because if there is a devil there is a God. Its a struggle to find good in the world I agree. The devil has to force himself on people and God doesn't have to try that hard because being a decent person is much easier. Just believe. That's what I think. However I still struggle from time to time. Best times in my life is when I had faith. Worsts times have been when I try to be a rebel like I don't need it anymore.