What does it mean when you dream of the devil and demons?

I imagine that depends on what those images mean to you, in the context of your own belief system. They would probably mean something different to a Christian than they would to a pagan or an atheist.

Our subconscious mind is responsible for dreams. Sounds like you might be fearing something, loss of friends, loss of faith, maybe feeling out of control in some aspect of your life, repressing something that you are in denial about in your waking life.

Dreams don’t mean anything.

The devil and demons probably represent something that is worrying you in real life. So perhaps you have a big project to complete or you think you are in trouble with a friend? The brain works in mysterious ways at times.

It means that I am beginning my rule over earth, and that your religion is wrong. Soon everyone will be possessed by Satan.

It probably means that you are watching too many apocalyptic movies, or reading too much of like literature.

Probably because you’re afraid of Satan.