Nothing to do with sarcasm. It means that if you claim to believe something, or if you think someone else believes something, you should start by acting as if it's actually true. Like when people hear "abortion is murder" and somehow end up parsing that as "I hate women" - don't do that. Or when people claim to think "abortion is murder" and yet don't act like millions of children are being murdered, but act as if it's a minor inconvenience to be settled with ballots.

As the title said: take ideas seriously. Don't use them as attire or status markers, to identify which political group you're in, or for fun philosophical discussions you ignore in real life. Did you know 2/3rds of self-described vegetarians claim to have eaten meat in the last 24 hours?