I've had a number of discussions with people of my parents' or grandparents' generation who consider themselves faithful to the Church's teaching who have uttered rank heresies: we have to earn our salvation through good works; unbaptized babies will go to Hell; only Catholics can go to heaven. When I try to correct their mistaken notions, they reply defensively, "Well, that's what the nuns taught us when I was growing up."
My interlocutor here is invoking the spotless reputation of the "pre-Vatican II nun," in full habit, always faithful to the Church's teaching, praying for our souls whenever she isn't teaching our children, the bulwark of the local parish in the days before the Council allegedly turned everything upside-down and changed the Church's teaching, etc. Surely Sister wouldn't have taught us something that wasn't so? She wasn't like, you know, those nuns we have today.
First of all, there are many good and holy and faithful religious sisters today, just as there were then. And there are heretical and unfaithful nuns today, just as there were then.
We also have to consider the possibility that you have remembered incorrectly or you initially misunderstood what it was that the sisters taught you. Maybe you took their exhortations toward good works to mean that they are the mechanism by which we are saved, instead of that by which we are built up in holiness and closeness to God. Maybe you mistook the theological theory of the Limbo of unborn babies to be a hellish place. Maybe you thought when Sister talked about all the benefits of the Catholic faith (the grace of the sacraments, the fullness of the truth), you thought she meant that without these things it was impossible for anyone to be saved. Perhaps that was it?
And then there's this, a thought quite likely anathema to many: perhaps Sister taught you wrong. Maybe she was too stringent in her theology. Maybe she went beyond what the Church officially taught and believed. Maybe you weren't taught what you should have been. We could give the benefit of the doubt and assume a good intention, though. Perhaps Sister, living in a predominately Protestant country that openly discriminated against Catholics, got a little defensive and pushed a little beyond what the Church taught, in order to distinguish "us from them" and establish a firm identity with firm teaching: "No! Earn your way to heaven! Only baptized babies can get in! Only Catholics!" A bad result, but people can be excused at least a little for what they do when their backs are against the wall; or if not excused, at least we can sympathize.
Now, you might say to me, "Nick, you're a post-Vatican II child, you don't know all the changes that happened! That's what the Church used to teach! Things ain't the way they used to be."
Dude. I can read.
I've read theology manuals from before the Council, the ones used in seminaries and universities. They do not say that we earn salvation by works. They do not say that unbaptized babies go to Hell. They do not say that only Catholics can be saved. Indeed, they say pretty much exactly what the documents of the Second Vatican Council and subsequent teachings say. There are shifts in tone and emphasis and certain thoughts are developed more, but those are not substantial changes. The faith is essentially the same as it ever was, expanded and deepened but never contradictory.
If Sister taught you those things back then, she was wrong. Let's presume the error is in your memory and not in her instruction. Yes, the Church looks rather different on the outside in many ways compared to then. People hear things put in a different way than when they grew up, and they wonder what the change was about, and why it happened, and they long for the certainty they once had when Sister taught them such hard and fast doctrine. But let's make sure, in our search for certainty, that we're not certainly wrong.