"Then i saw an angel coming down from heaven with the key to the bottomless pit and a heavy chain in his hand". verse 1 is john explaining how he saw an angel fly down from heaven in his vision.john is explaining the beginning of his vision which is the beginning of the end of the world.(think about revelation last chapter in the bible).god shows john how the world ends but this is just when the devil gets dealt with when everyones in heaven.so an angel comes down with a key (to the bottomless pit) and a heavy chain in his hand who knows whats going to happen next.read the next chapter and see how john explains his vision and how he describes. (don't worry i felt the same when i read this).

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REVELATION 20 (the thousand years)
Spiritualto those who aren't willing to read the bible.