I think the problem is a matter of definition, and a conflict in common uses of the word:
1. Formally a Catholic in the eyes of the church.
2. Calls themselves a Catholic
3. Is Catholic in their beliefs.
The last has a lot of grey areas as its not clear what you need to believe. There is no formal definition. its clear you do not have to agree with the Church on every single thing. On the other hand at some point (e.g. not accepting the trinity) you are seriously at odds with Catholic beliefs.
The first two definitions might sometimes include atheists.
1. Formally a Catholic in the eyes of the church. 2. Calls themselves a Catholic 3. Is Catholic in their beliefs.
The last has a lot of grey areas as its not clear what you need to believe. There is no formal definition. its clear you do not have to agree with the Church on every single thing. On the other hand at some point (e.g. not accepting the trinity) you are seriously at odds with Catholic beliefs.
The first two definitions might sometimes include atheists.