“have been by no means deprived of significance and importance in the mystery of salvation. For the Spirit of Christ has not refrained from using them as means of salvation which derive their efficacy from the very fullness of grace and truth entrusted to the Church” (Paragraph 3).
That same paragraph goes on to explain in deeper detail the background of the statement, “What exactly does this mean? Does this mean that Protestants cannot receive salvation?” While I can understand that the sentence may seem divisive, especially if one focuses on the subject of the sentence (“The Catholic Church”), looking instead at the object of a preposition (“means”) provides more insight.
“the gates of the netherworld shall not prevail against it” (Matthew 16:18).
The importance of what this means cannot be understated: Christ assigned to the Church—the apostles and their successors—the responsibility to spread the Word around to as many people as possible to ensure people would not have to face the gates of hell. This doesn’t mean that all outside the Church will face the gates of hell, but that the Church was founded to guide people away from the gates of hell.
In other words, do not think this statement means the Catholic Church is elevated, as some kind of reward; no, it actually says the opposite. We are tasked with a responsibility. Catholics believe the Church has a duty to present the truth of Christ to people.
This leads to the question about the “whole picture.” Of course, the Church believes it teaches the truth. I believe that most other denominations feel the same way (about their beliefs, that is). Put differently, there are by definition things that Presbyterian doctrine states that are not taught in the Catholic Catechism. And there are beliefs held by Catholics, that, for example, Lutherans do not believe. Thus it follows that if someone believes what the Catholic Church believes is true, they do not believe everything the Episcopal Church (for example) teaches as truth. However, they should never think themselves superior for this.
Now, if we accept the definition of truth as “the property of being in accord with fact or reality,” and using Aquinas’ words on truth—”And therefore, as good is convertible with being, so is the true” (Summa Theologica, First Part, Question 16, Article 3)—we see a correlation between goodness and truth, but this applies to the information itself, not to the person who holds the belief.
By way of analogy, let’s analyze the statement: “These violets are blue.” If person A believes the violets are pink, and person B that violets are blue, it follows that one person’s knowledge/information is more “good” than the other. But it would not follow that they are better as a person because they happened to know the truth about the flowers. Nor would it stop them from enjoying the flowers if they ever are presented with them.
This same logic applies to doctrine. If Catholics believe their information is correct, they should believe their faith to be “good.” And if they are being honest, they must also believe that someone who believes differently has faith which is less accurate. But even if this means I may believe my father’s Lutheran beliefs to be less factual, I, of course, don’t believe him to be less good than I, nor do I think this would ever stop him from being saved.