How do I tell my parents that I don't want a church wedding?

I was raised in a very strict Catholic family. We went to church every Sunday, the house is filled with crosses, pictures of the Pope etc and my family is devoutly religious. I have a great deal of respect for Catholicism, but after a lot of thinking and reading, I cannot honestly say I am a believer. I am, and have been for a while, an atheist. My girlfriend and I are getting married next year and I don't want a church wedding. However, growing up in such a strict Catholic household where church weddings are simply the done thing (they once went to an atheist couple's wedding held on a beach and came home and for years have criticized the couple and the service), I don't know how to tell them. I have never told them I am an atheist given how much they openly hate atheists. How do I tell them that I don't want a church wedding and that the reason is because I don't believe in God?