This alludes to the greatest favour of Allah to man; which is the raise of a Messenger who is by nature a man as any other man, but inspired with Divine inspiration and with a three fold task on account of people: 1) To rehearse unto them the revelations and signs of God through the verses of Qurän. 2) To sanctify them. 3) And to teach them the Scriptures and Wisdom, entering in people's heart the facts of God's true religion.
All the world in general, and all the people of the Arabian Peninsula in particular, were in manifest error before the advent of the Messenger of God. Mohammad (AS). All were struggling in the misfortune and calamities of ignorance and moral defilement when the Messenger of God started his heavenly mission, and recited to people his Message that was from God!
As to the question; why the BELIEVERS are addressed here, while the prophet's message was for all mankind? We say that it is the believer who gains from the prophet, and makes use of his message, and has allocated the prophet's Doctrine to himself.