Belief has a lot of will power behind it, which is why many incorporate their spirituality with magick--it fortifies it. The only thing that can cause someone's faith to waiver is doubt. But that can very well backfire because generally when someone doubts their faith, they just stop believing in everything all together. It's so ingrained in their mind (usually by family and spiritual leaders) that their religion is the only true religion.
The only people I have ever "changed" the religion of were those that already doubted their own and were interested in mine.
But that is within my own experience. I can't say I've ever desired to convert someone. Christians generally do that and some Satanists (ironicaly). The other religions (including pagan religions) are not like that.