The CCC (no. 837), under the title of "Who belongs to the Catholic Church?", teaches that full incorporation into the Church is possessed by those who "by the bonds constituted by the profession of faith, the sacraments, ecclesiastical government, and communion, are joined in the visible structure of the Church of Christ". This of course means that others, without being fully incorporated, still belong in some way to the Church. All of those in fact who have received Baptism begin to live the life of Jesus Christ and become *de facto members of the People of God (cf. c. 204). But non-Catholic Christians, though participating in the life of Christ, do not enjoy all the benefits or means of sanctification that Jesus wished his followers to have, for the completion of their growth in him (cf. Eph 4:12-13). The fullness of membership in the Church founded by Jesus, with all the immense benefits it brings, is only had by Catholics.
Membership of the Church is something to be taken advantage of, bringing as it does the right to direct access to Christ, achieved through full communion in the teaching, worship, sacraments and organic unity of the life which he sustains and secures.
And yet the christian life of many "good" Catholics too often remains weak because they make poor use of all their "membership advantages" within the Church. They go to the sacraments little (perhaps specially to the sacrament of Penance which, along with Holy Communion, is one of the only two that can be received frequently). They have no particular interest in reading the main documents of the Magisterium (those of the last Council, for instance, or the Encyclicals of the Popes), even though perhaps they do find time to read other books on doctrinal matters that may do them little good and even actual harm
The state of "non-practicing" members is one of special concern. While their neglect is often simply due to ignorance, it means they are deprived of many graces and strengths in the struggle against sin and the devil that all of us are faced with. It is a special apostolate to try to stir up in these people an awareness of what they are missing.
Can membership be *lost? The character of Baptism is never effaced from the soul; it remains there, even in a soul in Hell. But communion with the Church, external or internal, can be lost. While some Christians - Catholic or Protestant - lapse completely from their faith, there is an important difference to note. A Catholic's faith enriches him more than does that of a Protestant, and enables him to discover more concrete and immediate expressions of the extent of God's love and mercy: in his Real Presence, truly offering himself to us and for us in the Eucharist; in his voice - that of the Good Shepherd (cf. Jn 10:14) - really speaking to us with steadying guidance in the Church's authority and magisterium ("whoever listens to you, listens to me"); in his merciful word being spoken to us and healing power touching us in the sacrament of Confession.
If a Catholic lapses, therefore, he ignores, neglects or rejects much more than the lapsed Protestant. If grace touches him to become aware of the extent of his loss, this can spark off a strong desire to return, to make up, and to recover so many wasted opportunities.
*Non-membership. What are we to say about those who, lacking Baptism, are not members of the Church in any of the senses explained above? Here it is necessary to clarify the meaning of an old theological principle whose interpretation has caused difficulty in the past: "extra Ecclesiam, nulla salus": literally, 'outside the Church, there is no salvation'. Some people have wished to understand this saying in the most literal sense: i.e. that the person who is not formally a practicing Catholic, cannot be saved. The Church has condemned such an interpretation (DS 3870-3873).
This is not to say that the maxim is false. Properly understood, it is quite true. The Latin word "extra" can mean either "without" or "outside". The correct interpretation and sense of the maxim is that we cannot be saved *without the Church. It is *through the Church, which carries on and makes present the salvific work of Jesus Christ in the world, that all who are saved reach Heaven (even if it is perhaps only there that they realize it. Those who, through no fault of their own, have never known Christ or his Church, can still be saved. But their salvation too is also the effect of Jesus working through his Church. In a positive sense, this theological principle "means that all salvation comes from Christ the Head through the Church which is his Body" (CCC 846).
But this cannot make us indifferent to the situation of those outside, saying that no doubt they are in good faith and so can be saved. While this of course may be so, objectively they are exposed to many more difficulties. Lacking so many sources of strength, they are weaker against the attraction of evil and the Devil and more exposed to the temptation of ultimate despair (cf. LG 16). Therefore the Church feels urged to intensify her missionary activity towards them (cf. AG 8).
That mission of the Church involves all of us. A Christian who does not understand Christ's words about being salt and light and leaven, has little sense of world's need, and of his or her own mission. Vatican II reserves strong words for our possible responsibility here: "Believers can thus have more than a little to do with the rise of atheism. To the extent that they are careless about their instruction in the faith, or present its teaching falsely, or even fail in their religious, moral or social life, they must be said to conceal rather than to reveal the true nature of God and of religion" (GS 18).
The consciousness of what it means to be a member of Christ's Church should evoke a personal response of gratitude and increased responsibility. Belonging to the Church is not like being a member of a Club. It is not a weekend commitment. It is participation in an ideal and a movement. A living member senses his or her share in the Church's whole mission. Lay people "in particular ought to have an ever-clearer consciousness not only of belonging to the Church, but of being the Church" (CCC 899).