It sounds like a 14 year-old whose gone down the drama-queen avenue and listened to Marylin Manson record's without realising that it is a joke.
The only creature on this planet capable of feeling more sorry for itself than a 14 year-old is a right-wing white American male. To listen to either of them you'd think they were the most deprived and discriminated against of all animals.
And there's nothing like 'coming-out' as a Satan-worshipper to spook people.
Don;t over react, or you will find that a 14 year-old is playing you and having fun with it. She probably knows your fear of demons and the devil. The good news is that you need have no fear of such things. They don't exist.
What are here grades? Same as ever or getting worse?
Is her weight stable?
Have you seen her partially clothed recently, to an extent whwre you could see if she was cutting herself or such like (number of sel-harming teenagers I've seen is nasty)?
Has she ever been in trouble at school or with the police?
Does she do what she's expected to do around the house?
Does she have suspiciously large amounts of cash or possesions you don't know how she afforded?
Are her sleeping patterns normal?
Does she ever seem "out of it", as though she were drinking? Or behave in strange waysm, perhaps finding everything amusing?
Do you ever smell smoke on her?
Are her pupils ever massively dilated?
Does she find it impossible to get out of bed?
If she just says she loves Satan, but gets good grades, has no nasty scars on her arms, and is showing no signs of other disfunctions or harmful/illegal behaviours... let her have her teenage conceits.
If there are OTHER signs she's not coping well, talk to het and consider getting professional advice.
Remember though, due to your beliefs you will tend to over-react to her behavius, and she likely knows this.