I'm pretty sure I read this at the end of high school or perhaps early in college, but I only remembered a small part from the beginning. I had remembered liking it quite a bit and remembered that it had some kind of religious message. So I was happy to re-read it.
Mark Twain is a most excellent writer, so the book was well written, and full of Twain's wry commentary on the foibles of the human condition. In some ways, it was a dark story, essentially the story of Satan's visiting a small town in Austria in the sixteenth century, and befriending three young boys there.
There is lots of food for thought for those of us who grew up in the Christian tradition, and those of us who somehow think democracy is democratic, as opposed to being controlled by a small, powerful minority, a minority with the strongest lungs in Twain's telling. It's a short read, a novelette, actually, and is, of course free thanks to its having been published prior to 1923 or there abouts (1916, I believe Wikipedia told me). Go read it and stop wasting time on vapid pseudo-reviews.