Although it is considered rude to answer a question with a question, the best answer I can give to your question is this, “What do you think?”
First of all, let’s make a little clarification.
There is a difference between being considered a Catholic and being “baptized”. Baptism confers upon us a spiritual seal which is permanent. One can not remove or undo Baptism.
Some people, especially those who convert to non-Christian religions or become atheist, may want to be unbaptized. But, according to my understanding of the Catholic Faith, that is impossible.
If you’re asking because you are a non-practicing Catholic. You’re still Catholic. Not a Catholic in good standing, but a Catholic none the less.
So, back to the question.
I suppose you’re asking because you were Catholic and became Protestant or Muslim or atheist. So, what do you call yourself? If you call yourself Baptist, Calvinist, Hindu, atheist or whatever, and no longer call yourself Catholic, then you have “formally defected” from the Catholic Faith. You are no longer Catholic. You are a former Catholic.
That, is the rule of thumb. Once you have enrolled with and begun to function in another church or faith, you have thereby formally rejected the Catholic Church and are no longer considered Catholic by those who know you. Then, you are no longer a Catholic.
One last qualification
Let me rephrase the question. “If I was baptized a Catholic, am I still considered Catholic by the Catholic Church?
The answer to that question is, “Yes”.
1. Your records have not been expunged.
2. There is no mechanism for expunging of anyone’s records.
Barring anything crazy, like a fire or earthquake that destroys all records, there should always be a record, in the Church, recognizing you as a Catholic.
In order to prove to the Church that you have defected to another faith, you must meet the following conditions:
1. Make an internal decision to leave the Church. You’ve done that.
2. Demonstrate that you have left the Church. You have begun another life and are considered non-Catholic by your friends. You’ve done that.
3. Inform someone in the Catholic Church hierarchy. The Pastor of the Church where you were baptized is the best choice.
Since you have already done the first two and your neighbors all recognize you as something other than a Catholic, you can bring a letter or some other proof to the Catholic Church where you were originally baptized and you will meet the third condition as well. At that point the Catholic Church will have an official record that you are no longer Catholic.
The letter or other proof, should be as specific as possible, in the event of any Canon Law issues in the future.
Caveat, many people who ask this question are going through the annulment process. If that’s the case, you really need to talk to a Parish Priest or a canon lawyer, as well.
And finally, when you decide to come back.
If you were a Catholic who was not in good standing and you want to change; or if you had “formally defected” by simply beginning to live another faith life without legally informing the Catholic Church and you want to return; the first thing to do is to go to your Parish Priest. He will probably advise you to simply go make a Sacramental Confession. It’s that easy.
But, subject to what your Priest tells you, if you recorded your “formal defection” by some legal record, then he will probably advise you to make a legal record of your return. In other words, in addition to making your Sacramental Confession, write a letter documenting that you have repented of your defection and have returned to the Catholic Church and have it put in your Baptismal record next to your letter of defection.
For further information, try these links:
If I marry outside the Church
Heresy, Schism, and Apostasy