Prove all things (παντα [δε] δοκιμαζετε). Probably δε (but) is genuine. Even the gift of prophecy has to be tested (1. Corinthians 12:10; 1. Corinthians 14:29) to avoid error. Paul shows fine balance here.

Hold fast that which is good (το καλον κατεχετε). Keep on holding down the beautiful (noble, morally beautiful). Present imperative κατ-εχω (perfective use of κατα- here).