I would start by simply making your main character a devout Catholic and going from there. I would also caution you to make sure you have a story worth telling and not just to evangelize. I’ve read the Narnia books, which were filled with Christian imagery and allegory, but also had a genuinely good story narrative. They were story first, Christianity second, but only by a little. I’ve also read His Dark Materials, which were created to evangelize for atheism, and even if I didn’t disagree with the message, they weren’t very good. They were atheism first, story second. I would also say that if you are going to use allegory, don’t be afraid to be a bit subtle, most readers are far more intelligent than you might think, they’ll get the message without you beating it into them with a stick.