We know that hell is not empty from the following passage (among others):

Revelation 20:13-15 (RSV) And the sea gave up the dead in it, Death and Hades gave up the dead in them, and all were judged by what they had done. [14] Then Death and Hades were thrown into the lake of fire. This is the second death, the lake of fire; [15] and if any one’s name was not found written in the book of life, he was thrown into the lake of fire.
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