Isaiah 45 is addressed to Cyrus, king of the Persians, whom God calls His anointed — His messiah, His christ — the one who would shepherd His people. God tells him that He will give him the nations so that all will know that Yahweh is God and there is no other, so that all will know that He is the one who declares the end from the beginning and salvation to the ends of the earth. This is fulfilled initially during the time of Ezra, Nehemiah, and Esther when Cyrus was given the nations to rule and released the Jews from their exile to return and rebuild the city of Jerusalem and the temple. Of course, this pushes beyond Cyrus and points ahead to Jesus, the Anointed, to whom the Father gives all the nations. It is Jesus who says, “Turn to me and be saved, all the ends of the earth!” and “To me every knee shall bow, every tongue shall swear allegiance.”
This salvation that is declared to the ends of the earth, to all the gentile nations, is set in motion by Jesus through his church in the book of Acts. He gathers them together to pour out His Spirit on them and to send them to be His “witnesses in Jerusalem and in all Judea and Samaria, and to the end of the earth.” This is just how the book plays out. The first seven chapters take place in Jerusalem, and then persecution arises with Saul and the Jews against the church and it is scattered throughout Judea and Samaria. After Paul’s conversion, he begins to push out, taking the gospel into Asia, Greece, and what is now Europe. This is still the charge of the church, to take the gospel to the ends of the earth so that “the earth shall be full of the knowledge of the LORD as the waters cover the sea.”