Debaptism

Most Christian churches see baptism as a once-in-a-lifetime event that can be neither repeated nor undone. They hold that those who have been baptized remain baptized, even if they renounce the Christian faith by adopting a non-Christian religion or by rejecting religion entirely. But some other organizations and individuals are practicing debaptism.
In addition to "de facto" renunciation through apostasy, or heresy, the Roman Catholic Church envisaged from 1983 to 2009 the possibility of formal defection from the Church through a decision manifested personally, consciously and freely, and in writing, to the competent church authority, who was then to judge whether it was genuinely a case of "true separation from the constitutive elements of the life of the Church ... (by) an act of apostasy, heresy or schism." A formal defection of this kind was then noted in the register of the person's baptism, an annotation that, like those of marriage or ordination, was independent of the fact of the baptism and was not an actual "debaptism", even if the person who formally defected from the Catholic Church had also defected from the Christian religion. The fact of having been baptized remains a fact and the Catholic Church holds that baptism marks a person with a lasting seal or character that "is an ontological and permanent bond which is not lost by reason of any act or fact of defection." Nonetheless, formal requests for debaptisms are made; in France, a man sued the French Catholic church for "its refusal to let him nullify his baptism." He had been "un-baptized" in 2000, and ten years later he demanded to have his name stricken from the baptismal records, a request granted by a judge in Normandy, a decision appealed by the church.
Some atheist organizations, such as the Italian Union of Rationalist Atheists and Agnostics and the British National Secular Society, offer certificates of "debaptism". Not even those who provide the certificates consider them as having legal or canonical effect. The Church of England refuses to take any action on presentation of the certificate. The Roman Catholic Church likewise treats it as any other act of renunciation of the Catholic faith, although for a few years, from 2006 to 2009, it did note in the baptismal register any formal act of defection from the Catholic Church, a concept quite distinct from that of presentation of such a certificate.
In the years 2010 to 2016, 12,442 people "debaptised" in the Dutch-speaking Flemish region of Belgium by formally leaving the Catholic Church.
Using a hair dryer, some atheist groups have conducted tongue-in-cheek "debaptism" ceremonies, not intended to be taken seriously.