The Forgotten Ones(The Forgotten Ones Chronicles: Book 1) by William J. Finnegan

This is the story about what happens when the human race becomes so
Barbaric, so greedy, and so evil, that God himself turns his back on
us, leaving us to fend for ourselves. In this world, evil reigns
unchecked, and it's up to Loughlin, Diana, James, and the rest of the
underground clans, to stop Walter, York, Lucien, and their army of the
damned. They must stop them from forever closing the gates of Heaven
to mankind, to unleashing Hell on Earth, and forever becoming, The
Forgotten Ones.