The Catholic Church is the one true Church–and apart from the Body, one withers and dies. But, it is our Baptism that unites us to the Church and our unity is maintained by faith. Thus, we break unity by apostacy, schism, or heresy–but we must be guilty of it. Some Baptize person who is in good faith, while not being explicitly Catholic, may be united to the Body and draw life from it. They are not part of the full, visible unity of the Body, but are united at least in some way. This has been said in many different ways (before and after the Second Vatican Council) including being united to the soul of the Church or being in partial communion.
The point is, anyone who is saved draws their salvation from the Body of Christ which is the Catholic Church. And they do so from the points of unity they have with her–because those elements belong to the Catholic Church alone. Such a person therefore is not drawing salvation from a false denomination, but from the Catholic Church (ie, the Body of Christ which receives salvation from its Head, Jesus).
Likewise, an unbaptized person who is in good faith (that is, they desire to surrender themselves to the will and wisdom/Logos of God) but lack it, are also united to the Church by their faith and draw life from her as well.
Evangelization is key both in Catholic places and amongst non-Catholics because we need to persuade people to earnestly seek the will of God and to follow that will rather than following idols, atheism, their own desires, or from simply being apathetic. Only those who choose to abide in the true love of God can be saved (the Church is that visible communion of love).