In John's vision, he see many angels at work.  He sees one flying in mid-heaven, that is, in the heaven or atmosphere that surrounds earth.

                The angel's mission is the preach the 'Everlasting Gospel' to every nation, kindred, tongue and people.

                            This is the first and only place in the Bible where an angel is commissioned to preach the Gospel.  In this Gospel Age only men who are redeemed can preach, but at the End Time, just before Jesus returns, an angel will preach the Everlasting Gospel.