(O ye who believe) meaning: Hudhayfa [Ibn al-Yaman] and 'Ammar [Ibn Yasir]! (If ye obey those who disbelieve) i.e. Ka'b [Ibn al-Ashraf] and his followers, (they will make you turn back on your heels) they will make you return to your old religion: disbelief, (and ye turn back as losers) duped for loosing both this worldly life and the Hereafter and also for receiving Allah's punishment.