One of those novels you don't expect much from going into but are pleasantly surprised afterwards. I've included a description from the back cover that does a good job of summarizing the book. Barrett is also the author of the Judge Dredd novelization (Stallone version).
From the back cover: "Captain Lars Haggart was a soul waiting to be reborn...but before that blessed event he has been inducted into the Arm of God Regiment fighting for the beleaguered Churchers on a newly colonized planet. Their foe -- demons who could pop into existence, slay and pop out of existencethe next instant. The demons were winning that war, sending their Unborn opponents back to limbo, driving the living colonists toward extermination. But this was no fantasy, no business of the religious imagination. The fight was real, blood was blood, and swords cut sharp, for the Unborn were very much alive. Haggart was aware that this was frighteningly contradictory but first he had to fight the demons on their own terms -- learn how to appear behind their lines and do to them what they were doing to the humans. An unusual science fiction novel of a space colony in deepest trouble and of aliens who knew planetary secrets that were never in anyone's Holy Book."