Shining Steel - Lawrence Watt-Evans

Godsworld was settled by Christian fundamentalists who sought to escape the sinful Earth. They were certain Earth was doomed, and spent a century in suspended animation to reach their new home.
But Earth survived, and now the heathens have found them. John Mercy-of-Christ is leading the resistance to the invasion -- but the heathens have far more powerful technology than the Godsworlders.