I expect they know from your application whether or not you are a Catholic. If you are not, don't worry, they are still interviewing you! Just make it clear that you are prepared to support and respect the Catholic ethos of the school and are willing to learn about the faith as you go along by asking questions and taking advice as necessary. They might ask about what you would expect from a Catholic school and how you would support it, for instance children attending mass, and learning and saying prayers during the day etc., expressing and learning about their faith in various ways, and living their faith in their school lives. If you are a Catholic, they will expect you to know stuff in more detail, such as having some ideas about Sacramental preparation.