Cyrus was a King of Persia, founder of the Achaemenian dynasty and the Persian Empire, he reigned 559-529 BC. He seized Media in 550, went on to conquer Lydia, and his crowning success was the capture of Babylon. He was finally killed in battle against one of the tribes north of his Kingdom.
In Isaiah 45:1-4 God selects Cyrus to open the gates of Babylon and conquer Babylon, which Cyrus did in 539 BC. God does this for the sake of His people who are exiled there.
In 2 Chronicles 36:22-23 God stirs up Cyrus to make this proclamation throughout his kingdom: "all the Kingdoms of the earth have been given to me by the Lord God of heaven, and He instructed me to build Him a temple in Jerusalem, in the land of Judah.
All among you who are of the Lord's people, return to Israel for this task, and the Lord be with you". So through Cyrus a total of 42,360 persons returned to Judah along with 5,469 gold and silver items, livestock and donations, as listed in Ezra, chapters 1 and 2.
The actual construction of the Temple began in the second year of their arrival in Jerusalem. Cyrus is mentioned in the Bible's books of 2 Chronicles, Ezra, Isaiah and Daniel.
Note: A clay cylinder was found in Babylon called the "Cyrus Cylinder." It is full of inscriptions giving record of Cyrus' capture of Babylon, and tells how the captives of Babylon were returned to their homelands. This is in line with the decree Cyrus issued in 538 BC, allowing the Jews to return home to Jerusalem. The cylinder is now in the British Museum.
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