Take heed (βλεπε). Keep an eye on.

Thou hast received in the Lord (παρελαβες εν Κυριω). Second aorist active indicative of παραλαμβανω, the verb used by Paul of getting his message from the Lord (1. Corinthians 15:3). Clearly Archippus had a call "in the Lord" as every preacher should have.

That thou fulfil it (ινα αυτην πληροις). Present active subjunctive of πληροω, "that thou keep on filling it full." It is a life-time job.