We speak of people "converting to Catholicism," but more accurately, one "enters into full communion" with the Church. To say that one enters into "full" communion shows that one is already in some manner, in communion with the Church. Which is exactly what the Church teaches.
Lumen Gentium, which means,"light of the nations!" (Catholic Matters-Neuhaus) The Church says that all Christians who are baptized and believe in Jesus Christ are "in certain but imperfect communion" with the Church. That "light" holds the nations together and is the foundation of them all throughout time. Anyone who understands this can't argue invalidity. The Catholic Church has been around since the beginning from which all other denominations have branched .
In this view, entering into full communion is a matter of perfecting the imperfect. This is hard for both other Christians to take, and Catholics to explain. In today's culture, one is supposed to say that all religions are more or less equal to one another. Whatever religion "works for you." But then the Catholic Church "breaks" society's "rules" by talking about the perfect and imperfect.
But what is the Church to do when we believe that we are the Church of Jesus Christ? Most fully connected and ordered through time. Not everyone accepts this however. Not by a long shot!
Another example of this is when we state that, "the Catholic Church is the body of Christ. There is only one Christ and so there can only be one body of Christ." There are many churches, and many denominations but there is only one Church.
I do not mean to argue that all Christians should become Catholic. The point of this is to show how Catholics view those outside of the church. I didn't know how this all worked out, and I just stumbled upon this in my reading, and thought I'd share it with you. It makes sense... Just like a lot of things about the Catholic Church. =D