The foregoing account of the Church and of the principle of authority by which it is governed enables us to determine who are members of the Church and who are not. The membership of which we speak, is incorporation in the visible body of Christ. It has already been noted (VI) that a member of the Church may have forfeited the grace of God. In this case he is a withered branch of the true Vine; but he has not been finally broken off from it. He still belongs to Christ. Three conditions are requisite for a man to be a member of the Church.
1.In the first place, he must profess the true Faith, and have received the Sacrament of Baptism. The essential necessity of this condition is apparent from the fact that the Church is the kingdom of truth, the society of those who accept the revelation of the Son of God. Every member of the Church must accept the whole revelation, either explicitly or implicitly, by profession of all that the Church teaches. He who refuses to receive it, or who, having received it, falls away, thereby excludes himself from the kingdom (Titus 3:10 sq.). The Sacrament of Baptism is rightly regarded as part of this condition. By it those who profess the Faith are formally adopted as children of God (Ephesians 1:13), and an habitual faith is among the gifts bestowed in it. Christ expressly connects the two, declaring that “he who believeth and is baptized shall be saved” (Mark 16:16; cf. Matthew 28:19).
2.It is further necessary to acknowledge the authority of the Church and of her appointed rulers. Those who reject the jurisdiction established by Christ are no longer members of His kingdom. Thus St. Ignatius lays it down in his Letter to the Church of Smyrna (no. 8): Wheresoever the bishop shall appear, there let the people be; even as where Jesus may be there is the universal Church". In regard to this condition, the ultimate touchstone is to be found in communion with the Holy See. On Peter Christ founded his Church. Those who are not joined to that foundation cannot form part of the house of God.
3.The third condition lies in the canonical right to communion with the Church. In virtue of its coercive power the Church has authority to excommunicate notorious sinners. It may inflict this punishment not merely on the ground of heresy or schism, but for other grave offences. Thus St. Paul pronounces sentence of excommunication on the incestuous Corinthian (1 Corinthians 5:3). This penalty is no mere external severance from the rights of common worship. It is a severance from the body of Christ, undoing to this extent the work of baptism, and placing the excommunicated man in the condition of the heathen and the publican". It casts him out of God’s kingdom; and the Apostle speaks of it as “delivering him over to Satan” (1 Corinthians 5:5; 1 Timothy 1:20).
Regarding each of these conditions, however, certain distinctions must be drawn.
1.Many baptized heretics have been educated in their erroneous beliefs. Their case is altogether different from that of those who have voluntarily renounced the Faith. They accept what they believe to be the Divine revelation. Such as these belong to the Church in desire, for they are at heart anxious to fulfill God’s will in their regard. In virtue of their baptism and good will, they may be in a state of grace. They belong to the soul of the Church, though they are not united to the visible body. As such they are members of the Church internally, though not externally. Even in regard to those who have themselves fallen away from the Faith, a difference must be made between open and notorious heretics on the one hand, and secret heretics on the other. Open and notorious heresy severs from the visible Church. The majority of theologians agree with Bellarmine (de Ecclesiâ, III, c. x), as against Francisco Suárez, that secret heresy has not this effect.
2.In regard to schism the same distinction must be drawn. A secret repudiation of the Church’s authority does not sever the sinner from the Church. The Church recognizes the schismatic as a member, entitled to her communion, until by open and notorious rebellion he rejects her authority.
3.Excommunicated persons are either excommunicati tolerati (i.e. those who are still tolerated) or excommunicati vitandi (i.e. those to be shunned). Many theologians hold that those whom the Church still tolerates are not wholly cut off from her membership, and that it is only those whom she has branded as “to be shunned” who are cut off from God’s kingdom (see Murray, De Eccles., Disp. i, sect. viii, n. 118). (See EXCOMMUNICATION.)
Source:
newadvent.org/cathen/03744a.htm
) Better yet, just download some of the free mp3 files they offer online. There are detailed explanations of the Catholic positions on a variety of controversial topics (Mary, Purgatory, etc.). When these apologists explained these themes clearly to me, it was like a huge bandage had been lifted off my eyes!