Reducing public money for churches by leaving them
Formally leaving a religious group may reduce its public funding and therefore contribute to church-state separation. States collect “church tax” take charge of church-leaving, for example, Austria, Denmark, Finland, Germany, Iceland, Sweden and Switzerland. However, in others like Poland, Italy, Spain, France, Britain, Ireland, Belgium and the Netherlands, the state has a say in the keeping of church records only where they fall under data protection laws.
In Germany, for instance, the state collects the tax but does not supervise its use by the hierarchy. Quite the deal. In 2009 the tax deductability of the church tax cost the German state € 2.94 billion in lost tax income.  Thus, leaving the church in Germany saves the state money and helps to separate the two. And even where there is no church tax, membership numbers can be used by religious organisations to justify both political and financial privileges, as is done by the state church in England.
Vatican tries to stem the outflow
♦ The first attempt to deal with the new situation of people wanting to leave the Church en masse was Benedict XVI's letter to bishops in 2006 on “The formal act of defection from the Catholic Church”, (Actus formalis defectionis ab Ecclesia catholica). This was a theological challenge to the state's practice of allowing anyone to leave the Church at will. The Church insisted that there must be a "reception of that decision [to leave the Church] by the competent ecclesiastical authority". One couldn'tdo it merely by filling out the appropriate government forms: only the Church could decide who might “defect”. And, of course, in countries where Church membership has no tax implications, the state has no civil rules for leaving the Church and the Church has a free hand to set the conditions for getting out.
Although “Catholic” means “universal”, the requirements for officially leaving this church differ from one country to another. The road is smoother where the Church has had to face legal challenges (as in Italy) and where the state offers an easy administrative process (as in Germany) because it keeps its own records of church membership in order to collect “church tax”. However, where the Church controls all the records and it finds itself largely unchallenged, (as in Poland) the bishops can lay down onerous requirements to leave.
♦ After an increasing number of faithful formally left the Catholic Church in the wake of the clerical abuse scandal, the Vatican changed the rules again. This was done by a new document, “Omnium in Mentem” which came into effect on 9 April 2010. It removed the dispensations introduced in 1983 and with them all references to formal defection. As explained by the Archdiocese of Dublin, “it will no longer be possible to formally defect from the Catholic Church”. This formal Church-leaving process has been abolished in Canon Law. 
Once baptised, you are bound by the laws of the Church for the rest of your life. There is no release. As laid down by the Pontifical Council, those who have left the Church cannot be absolved of the obligation to obey it. Otherwise, as Msgr. Gordon Read stated in his commentary, the Church would be “an association one could freely enter or leave”. This would “evacuate Church law of any force if it were sufficient for someone to declare formally that he was leaving the Church and thereby [was] no longer bound by its laws”. 
♦ And finally, attempts have been made to stem the exodus at the national level, of course, with the approval of the Vatican. Church-leaving in Poland threatens the traditional predominence of the Catholic Church there and the defections in Germany erode the “church tax” which brings the Catholic Church some 5 billion euros a year.  See Polish Bishops set rules for leaving the Church (2008) and German “church tax”: Court confirms the bishops’ “pay to pray” decree (2012). In both countries the national bishops' conferences issued special rules governing church leaving. First the Polish bishops (2008-09-27) and then the German ones (2012-09-20) proclaimed that those who have de-registered themselves are to be denied any sacrament except final absolution.
Excommunicated, but still CatholicIn light of the first two measures, an Irish group which had been offerering online help to those wanting to officially leave the Church has temporarily suspended its website.  It is now attempting to get a clarification from the president of the Pontifical Council as to whether Canon Law still allows people to officially defect. 
However, Msgr. Read's commentary is firmly in line with Catholic theology. “The Catholic Church considers everyone who is baptised its member. These people do not have to claim they belong to the Church in a census,” says Jiri Gracky, from the Czech Catholic Bishops' Conference . At least from a theological point of view, the Church does not recognise the state's census figures, even though they better reflect what people actually believe. Its own Church laws allow it to keep on inflating membership statistics by counting everyone who's been baptised. The Dublin Archdiocese denies this, but provides no proof. Similar claims by the Church of England have been found to be untrue.
According to Catholic doctrine, undoing a baptism is simply not possible. Any attempt to have the record of the baptism removed runs up against the brick wall of Canon 849 which asserts that baptism is “indelible”. Anyone who has been baptised remains bound by Church (Canon) law and, theologically speaking, is not able to leave the Church. Even those who left when formal defection was still possible were considered to have excommunicated themselves, but to have remained Catholics, nevertheless.
Like Islam, Catholicism considers renouncing the faith to be a “sin” and those who wish to leave are seen as violating the Church laws that still bind them. As a Slovak discovered, it's no use invoking any secular laws about freedom of religion: the Church regulates this according to its own laws.  The best that can be achieved is a notation appended to the baptismal record. However, even this is being challenged in Spain. There the legal issue has become whether the European Union's data protection laws (1995, amended 2003) indeed apply to Church records.
Regardless of theology, government census figures represent political reality and in that sense church-leaving can ultimately lead to more secularism, as a shrunken membership makes it politically harder to justify religious privileges.