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Excommunication (OSV Encyc. 1997)
Through the grace given in Baptism, we are united to Jesus living with his very life in souls. Nothing worse can happen than to lose that union with Christ, and with the Church in which he lives and communicates himself. It is urgent never to forget this, especially since our communion with Christ is under constant threat from our human tendency to sin.
Venial sins do not break that communion (although, unless combatted, they weaken our bonds with Christ). But serious sin deprives us of grace and the presence of the Holy Spirit. God's life is extinguished in the soul; the branch, with the sap gone from it, is dead even if still physically united to the vine, to the Body of the Church. Through repentance and the Sacrament of Penance, the sinner can return to full life-giving participation in Christ's life and the communion of the Church.
Certain particularly grave sins incur the penalty of excommunication. Over and above the loss of grace, this involves a break with the bonds that shape and hold together our union with Christ in and through his Church. The most notable effect of excommunication is being excluded from the sacraments (c. 1331, §1, 2º).
Automatic excommunication follows on certain offenses: apostasy, heresy or schism (c. 1364); the direct violation by a the priest of the seal of Confession (c. 1388); the procuring of an abortion (c. 1398). In most cases, excommunication can be imposed only after due process (cc. 1341; 1314).
As with all penalties in the Church, the purpose of an excommunication is "medicinal", i.e. to bring about the repentance of the sinner. Repentance makes absolution from the excommunication possible (CCC 1463). If a person repents, the excommunication will always be lifted; but until this is done, he or she is barred from receiving even the sacrament of Penance, and from all the other sacraments.
Apart from excommunication imposed by law - with its external effects - one should mention the possibility of "self-excommunication", whereby a person undermines the bonds (especially those of faith and governance) binding the Christian to Christ. This can be the most dangerous and insidious form of excommunication.