A Covenant with Death — Bill Wetterman

Twenty-five years in the future, at the height of its power, the Global Realm collapses from within, leaving Peacock captured by the enemy and Arthur Pendleton unable to regain the power he once held. Only a miracle can save the Earth, and none is in sight. The Pendleton children make a final stand in hopes of buying time to evacuate as many of the faithful to Mars before their enemies assume total control.