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Verse 8 in chapter 1 of Acts says, But you will receive power when the Holy Spirit comes on you and you will be my witnesses in Jerusalem, and in all Judea and Samaria, and to the ends of the earth.

This verse gives us an outline for the entire book of Acts.  Luke obviously took this verse from a first-hand experience and used it to structure the rest of the book.  Beginning in Jerusalem (chaps. 1 through 7) they would carry their witness through Judea and Samaria (chaps. 8 through 12) and would continue “to the ends of the earth” (chaps. 13 through 28). (Horton page 40)

The book deals at great length with Jerusalem, Judea, and Samaria but the ends of the earth is a much broader term.  It is quite appropriate for Luke to end the book on an open-ended note since the ends of the earth were not reached within the pages he wrote.  Paul began to see and witness to many unsaved Gentiles but there was a great deal of the earth not reached yet.  In fact, our charge to this day is to continue the great work our dear brothers started all those centuries ago.

The major purpose and structure of the book is clearly outlined in Acts 1:8 and carried through to the end of chapter 28, but the actual verse will not be completed until the Lord himself returns.