In Latin, an important use of the accusative case is with prepositions. Prepositions and the nouns or pronouns they go with show direction toward, time, the means by which something is done, where and when something is done, purpose, and various other kinds of directions.
As an adverb, the word "extra" in Latin is properly translated into English as "outside." However, the same word in the maxim Extra Ecclesiam nulla salus is a preposition which takes the accusative case. It is, therefore, more accurately translated into English as "without." Consequently, the maxim Extra Ecclesiam nulla salus is more accurately translated into English as "Without the Church there is no salvation."
Why is this so important? Because, for the Feeneyites, the maxim means that an individual who is not formally a practicing Catholic cannot be saved. The Church has condemned this interpretation (cf. Denzinger-Schonmetzer, 3870-3873).
As Pope John Paul II explains:
"The Council speaks of membership in the Church for Christians and of being related to the Church for non-Christian believers in God, for people of good will (cf. Lumen Gentium 15-16). Both these dimensions are important for salvation, and each one possesses varying levels. People are saved through the Church, they are saved in the Church, but they always are saved by the grace of Christ. Besides formal membership in the Church, the sphere of salvation can also include other forms of relation to the Church. Paul VI expressed this same teaching in his first encyclical, Ecclesiam Suam, when he spoke of the various circles of the dialogue of salvation (cf. Ecclesiam Suam 101-117), which are the same as those indicated by the Council as the spheres of membership in and of relation to the Church. This is the authentic meaning of the well-known statement 'Outside the Church there is no salvation.'" (Crossing the Threshold of Hope, pp. 140-141).