Latin for There is no salvation outside the Church. Basically, the doctrine was designed to teach that no one who is not Roman Catholic could be saved.
This is an old belief within the Church. It was stated by both Origen and Cyprian of Carthage. The fourth Lateran Council in 1215 stated "One indeed is the universal Church of the faithful, outside which no one at all is saved...."
When asked what precisely this meant, Pope Pius XII said that:
"Actually only those are to be numbered among the members of the Church who have received the laver of regeneration and profess the true faith, and have not, to their misfortune, separated themselves from the structure of the Body, or for very serious sins have not been excluded by lawful authority."
Pope Eugenius agrees:
It (Roman Church) firmly believes, professes, and proclaims that those not living within the Catholic Church, not only pagans, but also Jews and heretics and schismatics cannot become participants in eternal life . . .and that no one, whatever almsgiving he has practiced, even if he has shed blood for the name of Christ, can be saved, unless he has remained in the bosom and unity of the Catholic Church."
Pope Boniface agrees:
With faith urging us we are forced to believe and to hold the one, holy, Catholic Church and that, apostolic, and we firmly believe and simply confess this (Church) outside which there is no salvation nor remission of sin . . . Furthermore, we declare, say, define and proclaim to every human creature that they by necessity for salvation are entirely subject to the Roman Pontiff."
However, this extreme view of the doctrine is not unanimously held. Pope Pius IX stated that:
In the worship of any religion whatever, men can find the way to eternal salvation, and can attain eternal salvation.
We must have at least good hope concerning the eternal salvation of all those who in no wise are in the true Church of Christ.
It should be noted that this dogma has been heavily qualified. The beliefs in baptism by desire and baptism by fire, for example, would seem to contradict it.
The Catechism of the Catholic Church explains:
"Outside the Church there is no salvation."
846. "How are we to understand this affirmation, often repeated by the Church Fathers? Re-formulated positively, it means that all salvation comes from Christ the Head through the Church which is his Body. . .
"This affirmation is not aimed at those who, through no fault of their own, do not know Christ and his Church:
"Those who, through no fault of their own, do not know the Gospel of Christ or his Church, but who nevertheless seek God with a sincere heart, and, moved by grace, try in their actions to do his will as they know it through the dictates of their conscience - those too may achieve eternal salvation."