If you are an atheist who was once a Catholic, you should consider having yourself excommunicated. There are few more definitive and public steps you can take on behalf of the rejection of religion. Unless and until you are excommunicated, you are still counted as a Catholic. Why would you allow them to regard you as even provisionally Catholic? What are you afraid of? Do you still have doubts about believing in God and are keeping your options open in case you want to return to the Church?

This appears to be the attitude of Catholic officials themselves and why they don't make excommunication easy. You can't just fill out an online form and get a certificate of excommunication, after all. Catholic officials may be hoping that former Catholics will eventually fear death enough to seek some sort of reconciliation with the Church. Until then, your name is used to indirectly contribute to the stature and power of the Catholic Church because they can report you as a member, thus boosting their perceived popularity among American religions.

Why Seek Excommunication?

Aside from denying the Catholic Church any more power than it deserves, there are a number of good reasons why a former Catholic should formally renounce their ties that organization.

The Catholic Church is an authoritarian and anti-democratic institution. Although this is bad enough for them internally, they also seek to encourage those principles externally in whatever community they can be found.

Politicians are sometimes threatened if they don't uphold public policies based on Catholic doctrine, but those would carry much less weight if Catholic leaders have fewer members to cite.

Politicians may feel that they have to adopt more Catholic policies to appeal to Catholic voters, but if the numbers of Catholics are reduced then politicians may feel freer to take positions consistent with secular values.

The Catholic Church encourages harmful policies and attitudes in society which need to be countered as forcefully as possible. Catholicism encourages misogyny and anti-gay bigotry for example, both of which have caused an incredible amount of psychological, emotional, and even physical harm over the centuries.

How Can I Get Excommunicated?

It is possible to be excommunicated in a specific, official pronouncement, but this is rarely done. More relevant is latae sententiae excommunication, or automatic excommunication, which can occur for the following reasons according to canon law:

1378: The absolution of an accomplice in a sin against the sixth commandment of the Decalogue ...incurs a latae sententiae excommunication reserved to the Apostolic See

1382: A bishop who consecrates someone a bishop without a pontifical mandate and the person who receives the consecration from him incur a latae sententiae excommunication reserved to the Apostolic See

1388: A confessor who directly violates the sacramental seal incurs a latae sententiae excommunication reserved to the Apostolic See

1398: A person who procures a completed abortion incurs a latae sententiae excommunication.

These can only be committed by a priest or bishop, so you can't use them.

1370: A person who uses physical force against the Roman Pontiff incurs a latae sententiae excommunication reserved to the Apostolic See

Excommunication isn't worth assaulting the pope, so this is out.

1367: A person who throws away the consecrated species or takes or retains them for a sacrilegious purpose incurs a latae sententiae excommunication reserved to the Apostolic See

Desecrating a consecrated host isn't as bad as assaulting a pope, but it's unlikely that you'd find a way to do this. That leaves one option:

1364: an apostate from the faith, a heretic, or a schismatic incurs a latae sententiae excommunication

You need to still be some sort of religious theist to be a heretic or schismatic, so if you are a secular atheist your only option is apostasy.

There is one more basis for excommunication:

1329: Accomplices who are not named in a law or precept incur a latae sententiae penalty attached to a delict if without their assistance the delict would not have been committed, and the penalty is of such a nature that it can affect them

So if one person does something that leads to automatic excommunication, any accomplices that were necessary to the act and who can have the penalty imposed upon them can also be excommunicated. This opens up options for ex-Catholic friends to find ways to help each other get excommunicated.

Process of Excommunication

If you want to be excommunicated, you have to go through official channels. Your local priest can't help you; instead, you must write a letter to your bishop.

Tell him where and when you were baptized (they won't excommunicate non-Catholics). Tell him of your apostasy; you must describe both an intention of apostasy and an outward manifestation. Apostasy doesn't count if you didn't intend it or if it didn't amount to anything. Explain that you know this means excommunication—ignorance of the penalty would get you off. State that you do not consider yourself a Catholic anymore and want your name taken off the official rolls of Catholics.

If you don't hear back after a little bit, send the letter again—but this time registered mail with a note that this is your second attempt. If you persist, you should be successful.