I can't tell whether this is deadpan humor or not.
I think closed borders will be considered a great evil in the future, but that's probably another way of saying that not enough people are agreeing with me now.
Well, I don't believe in purgatory, so that part was an (apparently ill-fated) attempt at deadpan humor. The question was sincere, though: if we're confident that we'll probably be scorned by future generations for something we're doing now, then the obvious response to that is to try to find out what it is, so we can do something about it now.
The closed-borders thing definitely has the features of a great candidate: closed borders are generally considered necessary, and you can make a reasonable case for them being evil.
Here's the new thread for posting quotes, with the usual rules: