In Italy there is no tax based on church membership, but rather a voluntary direction of 0.8% (otto per mille) of one's income tax to either the religious organisation of one's choice or else to cultural intitiatives of the state. Thus formal church membership has no tax implications and it is not recorded by the state. However, the state has become involved in obliging the Church to record people's wish to leave it.
Until a few years ago the Catholic Church refused to allow people to officially register their desire to leave it, claiming that it was impossible to “cancel the divine seal of baptism” (see Canon 849), and “to intrude in a foreign country's legislation”.
The Italian Union of Rationalist Atheists and Agnostics (UAAR) appealed to the Italian Data Protection guarantor to get the Church to acknowledge the process. On 13th September 1999, the guarantor ruled that a baptism could not be cancelled, as it “documented an event that had actually happened”. However, it established for the first time the possibility to have it officially noted by the parish that you no longer wish be considered part of the Church – or, in terms of the baptism ceremony, that you no longer wish to belong to the Church, figlio della Chiesa, but rather, wish to belong, as you did before your baptism, to yourself, (see Canon 1269). The priest is also obliged to give a response in writing, if so requested, that the notation has taken place.
However, the struggle to make it easy to leave is ongoing and “UAAR recently uploaded a new letter with stricter legal language to dissuade priests from requiring in-person visits or notifying relatives of the debaptism”.
What to do
Debaptising yourself, or sbattezzo is easier if you know where you were baptised. All you have to do is send a recorded letter to the parish priest declaring your intention to no longer be part of the Catholic Church, and don’t forget to attach a copy of your ID card. You don’t have to explain why. The UAAR offer a form to fill out and send if you don't wish to write a letter. (There are also further forms if you were Confirmed or took First Communion in a different church.) If you never participated in a “Confirmation” or “First Communion”, you can send him a different form (used with some success already) stating that you were never a Catholic.
If you don’t know where you were baptised, you have to do some research, such as contacting the parish in which you were married (if in a Catholic church) or Confirmed or took First Communion (but only in or after 1984), and these records will tell you where you were baptised (if it was in Italy). If you were baptised abroad, then it probably is not necessary.
The UAAR says that if you still don’t get any satisfaction, you can complain to the guarantor and so far, all such approaches have been successful in producing the required data.
UAAR (Unione degli Athei e degli Agnostici Razionalisti) http://www.uaar.it/laicita/sbattezzo/
"Presentation of UAAR", http://ibka.org/en/articles/ag02/uaar.html
Nicole Martinelli, "Debaptism 2.0: Fleeing the Flock Via the Net", Wired, 7 June 2007. http://www.wired.com/culture/lifestyle/news/2007/06/debaptism
Many thanks to Graeme Hunter for this summary