The Catechism of the Catholic Church, following historic Christian theology since the time of the early Church Fathers, refers to the Catholic Church as "the universal sacrament of salvation" (CCC 774–776), and states: "The Church in this world is the sacrament of salvation, the sign and the instrument of the communion of God and men" (CCC 780).
Many people misunderstand the nature of this teaching.
Indifferentists, going to one extreme, claim that it makes no difference what church one belongs to. Certain radical traditionalists, going to the other extreme, claim that unless one is a full-fledged, baptized member of the Catholic Church, one will be damned.
The following quotations from the Church Fathers give the straight story. They show that the early Church held the same position on this as the contemporary Church does—that is, while it is normatively necessary to be a Catholic to be saved (see CCC 846; Vatican II, Lumen Gentium 14), there are exceptions, and it is possible in some circumstances for people to be saved who have not been fully initiated into the Catholic Church (CCC 847).
Notice that the same Fathers who declare the normative necessity of being Catholic also declare the possibility of salvation for some who are not Catholics.
These can be saved by what later came to be known as "baptism of blood" or " baptism of desire" (for more on this subject, see the Fathers Know Best tract, The Necessity of Baptism).
The Fathers likewise affirm the possibility of salvation for those who lived before Christ and who were not part of Israel, the Old Testament People of God.
However, for those who knowingly and deliberately (that is, not out of innocent ignorance) commit the sins of heresy (rejecting divinely revealed doctrine) or schism (separating from the Catholic Church and/or joining a schismatic church), no salvation would be possible until they repented and returned to live in Catholic unity.
Fulgentius of Ruspe
"Anyone who receives the sacrament of baptism, whether in the Catholic Church or in a heretical or schismatic one, receives the whole sacrament; but salvation, which is the strength of the sacrament, he will not have, if he has had the sacrament outside the Catholic Church [and remains in deliberate schism]. He must therefore return to the Church, not so that he might receive again the sacrament of baptism, which no one dare repeat in any baptized person, but so that he may receive eternal life in Catholic society, for the obtaining of which no one is suited who, even with the sacrament of baptism, remains estranged from the Catholic Church" (The Rule of Faith 43 [A.D. 524]).
Augustine
"Whoever is separated from this Catholic Church, by this single sin of being separated from the unity of Christ, no matter how estimable a life he may imagine he is living, shall not have life, but the wrath of God rests upon him" (Letters, 141:5).
Lactantius
"It is, therefore, the Catholic Church alone which retains true worship. This is the fountain of truth; this, the domicile of faith; this, the temple of God. Whoever does not enter there or whoever does not go out from there, he is a stranger to the hope of life and salvation. . . . Because, however, all the various groups of heretics are confident that they are the Christians and think that theirs is the Catholic Church, let it be known that this is the true Church, in which there is confession and penance and which takes a health-promoting care of the sins and wounds to which the weak flesh is subject" (Divine Institutes 4:30:11–13 [A.D. 307]).
Cyprian of Carthage
"Whoever is separated from the Church and is joined to an adulteress [a schismatic church] is separated from the promises of the Church, nor will he that forsakes the Church of Christ attain to the rewards of Christ. He is an alien, a worldling, and an enemy. He cannot have God for his Father who has not the Church for his mother" (The Unity of the Catholic Church 6, 1st ed. [A.D. 251]).
"Let them not think that the way of life or salvation exists for them, if they have refused to obey the bishops and priests, since the Lord says in the book of Deuteronomy: ‘And any man who has the insolence to refuse to listen to the priest or judge, whoever he may be in those days, that man shall die’ [Deut. 17:12]. And then, indeed, they were killed with the sword . . . but now the proud and insolent are killed with the sword of the Spirit, when they are cast out from the Church. For they cannot live outside, since there is only one house of God, and there can be no salvation for anyone except in the Church" (Letters 61[4]:4 [A.D. 253]).
"When we say, ‘Do you believe in eternal life and the remission of sins through the holy Church?’ we mean that remission of sins is not granted except in the Church" (ibid., 69[70]:2 [A.D. 253]).
"[O]utside the Church there is no Holy Spirit, sound faith moreover cannot exist, not alone among heretics, but even among those who are established in schism" (Treatise on Rebaptism 10 [A.D. 256]).