And likeable too, brother. Don't settle for anything less than the whole truth.
From one Aquinas fan to another, you might like this link
There's a risk in trying to be the warner. You risk warning people away from your belief just as much as warning people away from your target. Probably the greatest source of irritation is inaccuracy. Whenever you, or somebody like you, say something that is untrue, or taken out of context, or viewed to be twisted in any way you have invalidated your entire arguement, and may have put the last straw on someones back.
I say this next part with a (temporarily) altered world view. I am now assuming that you are correct, that HP is drawing kids to the occult, that the occult is very real and dangerous. I say that to let you know I am not taking this next advice lightly. If you really want to warn people you have got to have primary source knowledge, and plenty of it. Because if you can't speak from the belly of the beast, if your information is only coming from those that say they have this primary source knowledge (many of whom clearly don't) you will run up against people (like me) that do, and they'll tear your arguements to shreds. You're going to have to read it, watch it and listen to it. And "it" means the stuff you think is drawing people to the occult, and the occult itself. I realize there is risk involved, but the risk if you don't do that is that your arguements will have a foundation of sand and will be ineffective. You've got to know enough to know that the spells in HP consists of nothing more than waving a wand (that is made with griffin and phoenix parts) and saying rhyming stuff in Latin (like that's hard, Latin was designed to rhyme; that's how you know which nouns, verbs, adjectives and adverbs all go together), which is clearly not real magic. And you've got to know enough to know that sometimes when people make songs abut Aleister Crowley they're making fun of him (referencing Ozzy Osborne's song that gets him in so much trouble, anybody that's actually read Crowley know's just from Ozzy's pronunciation that Ozzy is mocking Crowley... treating him fouly as the rhyme goes). You've got to know that anybody that says they were involved in Wicca and that they were "close friends with warlocks" is lieing (I posted that answer last week). Basically, if you want to tell people about hell, you're going to have to at least take the $.02 tour and it would be better to spend a few days there and get to know the natives.
And don't forget, I'll be hounding you every step of the way. I'm sure my tendency to be drawn to stuff that certain Christians later find objectionable isn't going to end any time soon. And I will defend my hobbies from all misfeasance and malfeasance with great dedication and furious effort.