Debaptism is fun. You may have your printable Certificate of Debaptism framed and hanging in your office/bedroom/bathroom... But maybe you really do want to get away from your church and your faith. Some church organizations will maintain you as a member no matter how long it's been since you attend. In fact, if you were baptized as an infant, the church may keep you on the rolls even if you never stepped into that church again.
Formally breaking ties with a church can be difficult. It can be intimidating and uncomfortable to address these topics. There's also the threat of having people talking about you, especially when there is a real fear of prejudice against non-believer and apostates.
Getting away from your now-former church will likely depend on a lot of factors. It may be enough for you to simply not go to your place of worship or engage in activities of a religious nature, but if you feel that even your former associations may be mis-construed as continued support you will have to take some positive action.
But be prepared: It may be possible that there will be people who either won't understand or will feel threatened. Understand that your social standing and personal relationships (and all the future effects those have on your life) may be put in jeopardy. It may be highly dependent on how strong your particular faith group views apostasy: your Catholic family may disown you, your CofE friends may simply laugh, or your Muslim father may throw acid in your face. Weigh the costs to your well-being with your desire to stand on the principle.
The one thing about the Catholic church that can be said is that they are very well organized. They have a buearacracy that rival a large country, and with that they have forms and procedures to accomplish almost anything with in the churches domain. Leaving the church is no exception. Formally, it's called the "Actus formalis defectionis ab Ecclesia catholica" in latin. Translated it mean "a formal act of defection from the Catholic Church". Commonly called a Declaration of Defection. The process is not difficult, but does require some formal steps to be completed.
Thankfully, there is a website dedicated to making this process easy. It's run out of Ireland, but applies to any Catholic. The site is called "Count Me Out" and is located here: http://www.countmeout.ie
The site has an extensive FAQ and information regarding the act of defection, and I urge anyone contemplating defection from the Catholic Church to visit the site.
Defection Form: http://www.countmeout.ie/samples/DeclarationOfDefection.pdf
There is a formal process to leaving the Mormon Church and it requires 2 actions. They are a resignation and removal from membership records. Please refer to this website for more detailed instructions: http://packham.n4m.org/leaving.htm
All that is required to resign is a formal letter, clearly worded with no ambiguity, stating that you are resigning from the Church of Latter Day Saints. This letter should be send to the bishop of the ward where you currently live and should state the date you are resigning, typically "effective immediately". Once this letter has been received by the bishop, you can consider yourself officially Ex-Mormon.
Removal from membership records
The resignation letter can contain the request to have your name removed from the membership records, but depending on where your membership was recorded (i.e. you've moved) it may require further action.
Muslims do not baptize. While Islam does have ritualized bathing to wash away sins, these are clearly not analogous to baptism for salvation. Even the "ghusl", performed before adoption of Islam, is performed for other reasons in the life of a Muslim, and can't really be compared to baptism. Despite this, you may have debaptized yourself into Baptismal Debt and/or simple want to leave the Muslim faith.
Unfortunately, there does not seem to be any formal method of leaving Islam. This is likely due to the fact that apostasy in Islam is considered such a horrific sin that in some Islamic countries it is punishable by death. Be aware, this is NOT an exaggeration. Muslim to Christian converts have been executed, as have missionaries promoting conversion.
Technically, all that is required to defect from Islam is an act of apostasy. Denying the existence of Allah, the prophethood of Muhammad, or some central tenet of Islam are all likely candidates for committing an act of apostasy.
As for official recognition of leaving the Muslim faith, it may very well come in the form of a fatwa. While the Qur'an does not suggest specific, extreme punishments, the Hadith does. If this is a real concern in the location/family/Mosque where you live/worship, please consider alternative ways to protect your life.
Others (Baptist, Jewish, Episcopal,etc)
There may be as many ways to leave a church as there are churches. Many churches and congregations are not part of any formal, larger organization with a proscribed method for leaving. The good news is that if your church is at least legally incorporated, it should have some documentation about membership. By reading it's by-laws or constitution you may find a procedure by which you can have your association with the church dissolved.
Conclusion (and what next)
Leaving your faith can be a difficult and anxious experience even without it being done publicly. Taking your decision and conclusions back to your congregation can be out-right scary. Only you can be sure when, how or even if you should formally break from your faith-community.
But leaving doesn't have to be the end. These days there are lots of non-religious groups to help people find community, or a sense of connection they may be missing from their former religious community. Humanist groups, Ethical societies, Atheist groups, Secular coalitions, etc etc. are all just a search engine away. I would be surprised if you couldn't find one in your local area.
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