The Council of Florence (A.D. 1438-1445) From Cantate Domino — Papal Bull of Pope Eugene IV
“…It 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, but will depart “into everlasting fire which was prepared for the devil and his angels” [Matt. 25:41], unless before the end of life the same have been added to the flock; and that the unity of the ecclesiastical body is so strong that only to those remaining in it are the sacraments of the Church of benefit for salvation, and do fastings, almsgiving, and other functions of piety and exercises of Christian service produce eternal reward, 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.”
“The Church is One, Holy, Catholic, Apostolic, and Roman: unique, the Chair founded on Peter. Outside her fold is to be found neither the true fold nor eternal salvation, for it is impossible to have God for a Father if one does not have the Church for a Mother.”
VEN POPE PIUS IX
“There is only one universal church of the faithful, outside which there is absolutely no salvation.”
IV Lateran Council
“It is a sin to believe there is salvation outside the Catholic Church!”
VEN POPE PIUS IX
“There is no salvation outside the Catholic Church. Anyone who resists this truth perishes.”
ST LOUIS MARIE DE MONTFORT
“It must be held as a matter of faith that outside the Apostolic Roman Church no one can be saved, that the Church is the only
Ark of Salvation, and that whoever does not enter it will perish in the flood.”
VEN POPE PIUS IX
“There is no entering into salvation outside of the Church, just as in the time of the Deluge there was none outside the Ark which denotes the Church.”
POPE JOHN PAUL II
“I profess that outside the Catholic Church no one is saved.”
POPE SYLVESTER II
“It is impossible to be joined to God except through Jesus Christ; it is impossilbe to be united to Christ except in and through the Church; finally, it is impossible to belong to the Church except through the bishops who are united to the Supreme Pastor, the successor of Peter.”
POPE JOHN XXIII
“The Catholic Church alone is the Body of Christ, of which He is Head and Savior. The “People of God” and the “Mystical Body of Christ” are one and the same thing, both of them designating the Church. Membership in the Church requires conditions other than Baptism alone; it requires identical faith and unity of communion, so that by means of the Catholic Church alone, which is the unrestricted instrument of salvation, is it possible to obtain the fulness of the means to salvation. Indeed, the Church is both a sure and an exclusive means of attaining salvation. We must always remember the unity of the Mystical Body outside which there is no salvation, for there is no entering into salvation outside the Church. Outside this Body, the Holy Spirit gives life to no one: those who are enemies to unity do not participate in the charity of Divine Life; those outside the Church do not possess the Holy Spirit. The entrance to salvation is open to no one outside the Church!”
POPE PAUL VI
The difference between these quotes (and there are many, many more)and the comments in this thread are that these quotes have “authority”. So, believe whatever you want.