If you were ever baptized in the Church, they still count you as a member. It does not matter that you've been an atheist for years. As far as the Catholic Church is concerned, you are still one of them. It also does not matter whether you were so young at the time you were baptized that you had no idea what was happening. They'll keep counting you until you make your "defection" official.
The good news is that it is possible to leave the Catholic Church,. They might not make it easy, but it can be done. The process involves the following steps:
- Complete a declaration of defection.
- Send it to your local diocese.
- Realize that they may request a meeting to discuss your decision. You don't have to comply, but you do need to respond or they may use this as an excuse not to process your request.
Here is some additional information about how to quit the Catholic Church that seems geared toward atheists.
An early version of this post appeared at Atheist Revolution in 2010. It was updated to fix broken links, improve clarity, and provide additional information.