John, you’re right; someone who is truly filled with the Spirit ought to be able to cast out demons, handle snakes, etc. Christ said that if we had so much as the faith of a mustard seed could move mountains (and few people today who don’t grind their own spices realize that a mustard seed is hardly bigger than a fleck of dust).
But the sad fact is that, put to the test, very few of us indeed have enough faith for those things. Jesus wasn’t kidding. We may think we’re strong in the Word, that we have great faith, but not too many of us can stand up and challenge the Prince of Darkness himself. Proper humility ought to make us realize that we can barely resist the temptations to sin that the Evil One puts in our paths every day, much less get into a fistfight with him over someone else’s soul. I’d exercise caution before claiming the faith to cast out demons in His name.
I also think God knows me well enough to know that I just might move a mountain or two due to my inflated ego. And that wouldn’t be nice.