For centuries, the only Christian Churches were Catholic--Roman-Catholic, Orthodox-Catholic. "Outside the Church there is no salvation" was said during the third century, long before any other Christian denominations were thought up. St. Cyprian (third century) noted that people needed to learn and understand the doctrine of salvation from the only Church who was then on hand to teach the doctrine of salvation.
Christians are the Body of Christ. Christians have been taught the doctrine of Christ and Salvation. Back in the third century, there was only one place to learn this. The Church. It is now spread all over the world, all over the Internet by Christians. If St. Cyprian were alive today, do you truly think he would be saying, "The only place you can learn about the salvation/redemption Christ brought into the world is inside the Catholic Church"?