Because the text implies such.
And a measuring rod similar in appearance to a staff was given to me, saying, “Get up and measure the temple of God and the altar and those who worship in it. And leave out the courtyard outside of the temple, and do not measure it, because it has been given to the Gentiles, and they will trample the holy city for forty two months. Revelation 11:1-2
The courtyard and the city are left off from being measured, and are to be trampled. The temple, altar and those who worship therein are measured. The implication is that those things which are measured are not trampled. This makes perfect sense as it is a symbolic way of describing GOD's preservation of those who are his, i.e. those who measure up to his standard.
This is one reason I don't believe it is referring to Jerusalem's destruction in 70 AD. The verse would not make any sense if that were the case.
And a measuring rod similar in appearance to a staff was given to me, saying, “Get up and measure the temple of God and the altar and those who worship in it. And leave out the courtyard outside of the temple, and do not measure it, because it has been given to the Gentiles, and they will trample the holy city for forty two months. Revelation 11:1-2
The courtyard and the city are left off from being measured, and are to be trampled. The temple, altar and those who worship therein are measured. The implication is that those things which are measured are not trampled. This makes perfect sense as it is a symbolic way of describing GOD's preservation of those who are his, i.e. those who measure up to his standard.
This is one reason I don't believe it is referring to Jerusalem's destruction in 70 AD. The verse would not make any sense if that were the case.
This introduces another argument. That of identifying the "woman" of chapters 17 and 18.
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John saw a "woman drunken with the blood of the saints, and with the blood of the martyrs of Jesus," 17:6. The woman had this name written on her forehead: "MYSTERY, BABYLON THE GREAT, THE MOTHER OF HARLOTS AND ABOMINATIONS OF THE EARTH," 17:5. The angel said that "the woman" was "that great city," 17:18 in whom "was found the blood of prophets, and of saints, and of all that were slain upon the earth," 18:24. Then John wrote, "Rejoice over her, thou heaven, and ye holy apostles and prophets; for God hath avenged you on her… Thus with violence shall that great city Babylon be thrown down, and shall be found no more at all," 18:20, 21. So who was this "woman," this "great city?"
John gave us a clue in Revelation 11:8, where he wrote, "And their dead bodies shall lie in the street of the great city, which spiritually is called Sodom and Egypt, where also our Lord was crucified." This shows us that John was referring to the Jerusalem of his day. To prove this assertion, John wrote that Sodom also used as a figurative name. That means it does not tell us the actual name of the city but its spiritual condition. Once more, in letting the Bible interpret itself, we find this is a reference to Jerusalem. In Isaiah, chapter one, after declaring that he had a "vision…concerning Judah and Jerusalem", Isaiah wrote, "Hear the words of the Lord, you rulers of Sodom." In Jeremiah 23:14, because of the adulterous prophets, God said that Jerusalem and her inhabitants were "all of them unto me as Sodom."
In Luke 13:33-34, Jesus said, “Nevertheless I must journey on today and tomorrow and the next day; for it cannot be that a prophet would perish outside of Jerusalem. “O Jerusalem, Jerusalem, the city that kills the prophets and stones those sent to her! How often I wanted to gather your children together, just as a hen gathers her brood under her wings, and you would not have it!" Then, in Matthew 23:29-37, Jesus blasted the Jews of his day for killing the prophets and the apostles. He declared that they are the children of their fathers who also killed the prophets. Then in verse 32, Jesus said that they would complete the sin that their fathers started. But the most crucial evidence is found in verse 35, where Jesus said, "upon you (i.e., the Jews of His day) may fall the guilt of all the righteous blood shed on the earth." Then He said, "I tell you the truth, all of these things will happen to you people who are living now. Jerusalem, Jerusalem! You kill the prophets and stone to death those who are sent to you," 36-37. In both passages, Jesus told the Jews of His day that they were guilty of "all the righteous blood shed upon the earth" (see also Acts 7:51-52). Therefore, since both of these passages deal with the same crime and the same judgment, the "great city" of Revelation must be the Jerusalem of Christ's generation. So, Revelation was written during a time Jewish persecution upon Christians which had to be prior to 70 AD.