If I write a page about a current pop star that talks about what they're doing, that is a fact such as dedicating themselves to Satan or involved in witchcraft, can I be sued by them even though it's a fact?
(Not a lawyer)
In my country at least, the law is very clear: truth cannot be libel. However, for something to be considered "truth", you'd need undisputable proof, you'd need evidence, the truth would need to be attestable.
Now what would you intend to prove, and how would you prove it? If the person in question is a Wiccan or a LaVeyan Satanist, that's their religion, they have every right to follow it. Claiming that there's something wrong with following any particular religion is a whole other can of worms - it's religious discrimination and hate speech, and would get you sued on that ground.
If you intend to prove that the person "cast the evil eye" on someone, or caused milk to sour, or flew on a broomstick, that would be quite hard to prove, and would be quite a surprise to the scientific world if you succeeded.
Are you familiar with the legal notion of libel/slander/defamation?
Oddly enough, America has a much more relaxed attitude about libel, slander and defamation. This is why Elon Musk is getting sued by the Thai cave rescuer (whom he dubbed pedo-guy for rejecting his mini-sub rescue scheme).
In America you might be able to get away with it. But in other Western countries, you will have to prove your allegations in court to beat the law suits.
In 2019, you could be sued in multiple countries at the same time.
Anyone can sue you for anything. If you're a regular person being sued by a wealthy one, or a large business, you will be crushed, even if you're in the right.
It is legal to say things about public figures that are true.
But truth is about facts, not about your interpretation of what they are doing. If a celebrity is an out Pagan and you write about her/his involvement in Wicca, that is okay. If you think that being Wiccan is really worshiping the devil and you write that the celebrity is a Satanist, then it is potentially libel and you could be in trouble if the celebrity or someone who works for her/him decides to pursue it.