Dear Scott:

No matter how a person becomes a Catholic, to leave the Catholic Church is a Grave Sin.

As to whether or not this will constitute mortal sin depends. For a sin to be mortal three things must be true:

1) the sin must be grave (this one is)

2) the person must know that it is grave

3) the person must be able to exercise unimpaired free choice to commit the sin

It is very unlikely that a person who grows up Catholic, or becomes a Catholic in adulthood (regardless of RCIA) can claim that he does not know that to leave the Faith is Grave sin. I suppose it is possible in some weird way, but it is unlikely.

The third requirement for mortal sin is usually the one in question. Some people may leave the church because of mental defect, emotional duress, or some other circumstance that impairs their ability to make a free choice.

It is incorrect that a person who educates himself instead of attending RCIA is in invincible ignorance. There are many other ways of learning the faith than from RCIA. If the person is educating himself he should come across the Salvation only in the Church doctrine.

It should be noted that even if the RCIA class, or one's parents, never taught the person that leaving the Church is sin, that does not automatically let the person off the hook. The Church teaches that we have an obligation to seek the truth for ourselves. While our teachers will be held accountable, our own responsibility to seek the truth remains. Thus, this may mean in some circumstances a person is still culpable and cannot claim invincible ignorance.

Those interested in the faith should certainly be told that joining the Catholic Church is a permanent commitment and that to leave is a sin. But, whether or not the teachers teach that, all have a responsibility to learn the basic doctrines of the faith.

Since we have many resources on the Internet and with the Catechism of the Catholic Church there is little excuse.

I might add that the Church also teaches that willful ignorance, a desire to not read the Catechism, for example, in order to keep oneself in ignorance of the Faith INCREASES one's culpability.

Ultimately only God knows where the lines are drawn between degrees of culpability and invincible ignorance.

God Bless,
Bro. Ignatius Mary