I know it's not scifi but it's still cool.

216 is the number that Nicolae Jetty Carpathia (The Antichrist) uses in the series.

No one seems to know what it means and the authors aren't talking.

I've figured it out at long last.

666 is the number of the Beast in Revelation.

6x6x6 = 216

But that's not all

NICOLAE JETTY CARPATHIA
A=1, B=2, C=3, etc.
The numbers of his name equal 216

Finally,
The number of sealed believers in Revelation is 144,000
The number of the Beast again is 666

144,000 divided by 666 equals
216.216216216216216...