Rawb,
That’s a big topic. I’ll say a prayer for you as you journey in the RCIA. At the risk of giving too much, I’ll try to respond.
You are correct in that excommunication is an exclusion of a Catholic from certain spiritual, and sometimes temporal, benefits of the Church, but it is not a condemnation of a person’s soul. It impairs a person’s communion with the Church, but does not expel the person from the Church nor call for “shunning.” You are also correct in that only God can judge the ultimate fate of anyone. Excommunication is a last resort of tough love when a law or precept has been issued to protect a grave and sacred principle or value, and then that law or precept gives this as a penalty for its violation.
The Catechism summarizes from other sources. In the case of the paragraph 1463 you cited, the Catechism draws from the canon law of the Church. Canon 1331 is most relevant so I’ll quote it. But the entire fifth book of the code of canon law, Sanctions in the Church (cc. 1311-1399) would have to be taken into account. It is a very complex subject, so I’ll make some brief comments that I hope will assist you with basics.
Canon 1331 “§1. An excommunicated person is forbidden: 1º to have any ministerial participation in celebrating the sacrifice of the Eucharist or any other ceremonies of worship whatsoever; 2º to celebrate the sacraments or sacramentals and to receive the sacraments; 3º to exercise any ecclesiastical offices, ministries, or functions whatsoever or to place acts of governance. §2. If the excommunication has been imposed or declared, the offender: 1º who wishes to act against the prescript of §1, 1º must be prevented from doing so, or the liturgical action must be stopped unless a grave cause precludes this; 2º invalidly places acts of governance which are illicit according to the norm of §1, 3º; 3º is forbidden to benefit from privileges previously granted; 4º cannot acquire validly a dignity, office or other function in the Church; 5º does not appropriate the benefits of a dignity, office any function, or pension, which the offender has in the Church.”
The Church has the right to penalize its members (cc. 11, 1311) when they violate externally a law or a precept to which a penalty is attached. In general, a censure is an ecclesiastical penalty that deprives a Catholic of certain spiritual goods of the Church (c. 1312). Excommunication is a censure or a “medicinal penalty” designed to shock an offender into realizing the seriousness of the offense, reforming, and being returned to full communion with the Church (c. 1331). It is a response of the Church to certain violations of Church law or precept that are external and gravely imputably by reason of malice or culpable negligence (c. 1321).
There is some similarity between the notion of this grave imputability and the type of freedom, knowledge, and intention required in the case of mortal sin. Still, there is a general separation to be made between the external forum, what is publicly viewable and judgeable, and the internal forum (conscience), which is not subject to public inspection. Canon law generally pertains to the external forum. It does not judge conscience nor is it privy to conscience, only observable actions.
Sometimes excommunication can only be imposed as a result of some juridial or administrative
process in the Church by a competent authority (a so called ferendae sententiae penalty). But in other certain egregious situations, it is automatically incurred by the very commission of a certain action for which canon law specifies that as a penalty (a so called latae sententiae penalty). The distinction between these would be found in canon 1314.
If excommunication is imposed by a process (ferendae sententiae), the person is informed through a decree. If it is automatically incurred (latae sententiae ), a competent Church authority may also issue a decree stating that fact. So a person should know.
Now to understand the “certain ecclesiastical acts” mentioned in the Catechism, it would be easiest to reference the automatic excommunications to be found in certain canons.
Canon 1364 §1 apostasy, heresy, schism
Canon 1367 sacrilegious mistreatment of the Eucharistic species
Canon 1370 §1 physical force against the Roman pontiff
Canon 1378 §1 absolving an accomplice in violation of the Sixth commandment outside of danger of death (see c. 977)
Canon 1382 a bishop consecrating someone a bishop without a pontifical mandate and the person who receives the consecration
Canon 1388 §1 a confessor directly violating the sacramental seal of confession
Canon 1398 procuring a completed abortion
But it can also be inflicted through a process, so we couldn’t list all possibilties. In the case of the parish in Saint Louis, Archbishop Burke declared a case of schism that is easily found on the ‘net.
I’ll post the rest of the important considerations next.