That used to be the official teaching of the Catholic Church but the teaching has changed. The Unam Sanctum (a papal bull issued by Pope Boniface in 1302), says this:
"With Faith urging us we are forced to believe and to hold the one, holy, Catholic Church and that, apostolic, and we firmly bellieve 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."
Several popes and 3 councils confirmed this papal bull and it was an official Catholic dogma for almost 700 years..
Then along came Vatican Council II (1962-1965) and the Dogmatic Constitution on the Church, Lumen Gentium. All of a sudden, the definition of "Christian" included others than just Catholics. In fact, not only did the Catholic Church drop the requirement to be a Catholic, but now Moslems are specifically identified as believers, too:
"15. The Church knows that she is joined in many ways to the baptized who are honored by the name of Christian, but who do not however profess the Catholic faith in its entirety or have not preserved unity or communion under the successor of Peter. For there are many who hold sacred scripture in honor as a rule of faith and of life, who have a sincere religious zeal, who lovingly believe in God the Father Almighty and in Christ, the Son of God and the Saviour, who are sealed by baptism which unites them to Christ, and who indeed recognize and receive other sacraments in their own Churches or ecclesiastical communities…
"16. Finally, those who have not yet received the Gospel are related to the People of God in various ways. There is, first, that people to which the covenants and promises were made, and from which Christ was born according to the flesh (cf. Rom. 9:4-5): in view of the divine choice, they are a people most dear for the sake of the fathers, for the gifts of God are without repentance (cf. Rom. 11:29-29). But the plan of salvation also includes those who acknowledge the Creator, in the first place amongst whom are the Moslems: these profess to hold the faith of Abraham, and together with us they adore the one, merciful God, mankind's judge on the last day. … "
Here is a big example of the Roman Catholic Church effecting a change in matters of faith and morals. What happened in Vatican II wasn't just a clarification of existing dogma -- it was a complete 180° turn.