You’re always better off flattering an audience’s intelligence. If your audience smells even mild condescension, you lose their sympathy and may not get it back.
But most condescension — in writing — is accidental. The most common way of unintentionally trying your audience’s patience is to elaborate on a concept that your audience doesn’t need to have elaborated.
Just remember what it’s like to not hear what someone said, beg the speaker’s pardon, and then hear him or her not simply repeat the phrase you missed — say, “Big Ten” — but actually explain the whole concept, starting from first principles. This is frustrating when you didn’t say “I’m sorry; What?” because you don’t know what NCAA basketball is. You simply didn’t hear, period.
If you feel compelled to (re)explain a familiar concept, find a novel, surprising way to phrase it. Or just preface your explanation with some variation on “As we all know….”