I am confused...

We are told that in the last days when God has eradicated Satan and all his cohorts from the planet and all those who denied and rejected Christ have been destroyed, God will send a giant cube down to earth called New Jerusalem.

This city will be 1500 miles wide, 1500 miles long and 1500 miles high and be where we who have accepted Christ shall live. Within this place there shall be no more tears or fears or sorrows and only love and harmony will prevail.

Yet, we are ALSO told that on each side of this city shall be 3 gates, each guarded by a warrior angel who will ensure that only those whose names are in the Book of Life may enter into the city.

So this is where I am getting confused.

If Satan and all his demons have been vanquished and everyone who denied Christ has been sent into the fires of Hell, WHO are the Angels protecting the city from?

Have I missed something in the telling of the narrative or is there a rational explanation you could provide?

Thank you.

Andrew
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What are the angels guarding us from? - https://www.worthyask.com/question/what-are-the-angels-guarding-us-from/