Er... I think that the two magics are different. The power that the Istari had and occasionally used was God-given, rather like God giving the apostles the power to preform miracles in the new testament. However there are also sorcerers mentioned in the new testament that have certain powers that do not come from God and are condemned by Him. As for HP, well... the magic used there is of the latter type. Oh dear, I'm not making my point very well. I know my explanation here is rather unclear and full of holes but I really don't have the gift of explaining myself well. Sorry.

I would like to mention though that I have read the books and I enjoyed them (although I didn't like everything in them). I also have many other Christian friends who enjoy these books very much, so not all Christians are against them. Many think it's purely a matter of what's going to hurt their spiritual walk and if God would mind them reading it. It has nothing to do with narrow-mindedness or the religion of the author (at least not with the Christians I know). Please respect those who choose not to read them. I consider it a courageous decision to follow what you believe in, despite pressure from peers and society.