According to this article one is catholic if:
There are clear, classical and traditional requirements of being a Catholic, most recently expressed by Pius XII. He states that the members of the Church are those who:
- Are baptised
- Profess the faith
- Have not separated themselves from the body
- Have not been excluded by legitimate authority (viz. by a perfect excommunication by the Roman Pontiff)
Do the ones who separate themselves from the Church have to still follow the rules to exercise the validity of certain things?
For example: a "former" catholic getting married outside of the church. Is this marriage valid? As a catholic you have to get married with a catholic priest. But when you declare you aren't catholic, does this still apply to consider the marriage valid? Will the catholic church recognize this marriage as a valid marriage as it recognizes the marriages of non-Catholics?
Sub-question: How does one separate from the body of Christ(Catholic Church)?