English Translation
Spies make it their business to know things that you don’t want them to know.
Why is “von denen” used here instead of “die” for the things?
‘Von’ takes a Dative and the sentence is relative, it talks about something that was said before.
And the Dative relative plural pronoun is ‘denen’
That’s not easy to explain.
“die” would be the direct object of “wünschen”, which would mean you want the things for yourself. But that’s not what you want to say in this case. There’s something about the things that you wish, namely that the spies don’t know them, which is mentioned in the relative clause that follows.
In the english sentence, “want” does not refer to “things”, but rather to “them” in “them to know”, while “that” does indeed refer to the “things” as a direct object, but of the verb “know”.
German doesn’t use the “want them to know” construction, so it needs a relative clause. And as soon as you introduce the relative clause, “wünschen” would refer to “Dinge” as a direct object if you use “die” as the relative pronoun. So you need “von denen” to express that it’s about some aspect of “Dinge”.