My title explains itself. But it's misleading. There are more than forty reasons. In fact, there are at least ten to the 82nd power, which, I am told, is the number of atoms in the universe. And that's just in ordinary matter, which makes up only 4.9% of the universe, the rest being dark matter and dark energy. Each of my reasons is an independent point, so I have not organized this book by a succession of chapters or headings. After all, most people only remember a few big ideas or separate points after reading or listening to a book. I've never heard anyone say Oh, that was a good continuous-process-of-logically-ordered-argumentation but I've often heard people say, Oh, that was a good point. Which takes me back to my main point: Why are you a Catholic? is a good question. A good question deserves a good answer. Here are forty of mine.