Codex Babylon by Robert Kroese

A secret cabal of demonic forces threatens to destroy civilization and plunge the world into a new Dark Age. Humanity's only chance is a shadowy organization called GRAIL--the modern day heirs to the Knights Templar--which has discovered the secret to time travel. The past cannot be changed, but if GRAIL can send an agent into the past to recover a lost book on demonology called the Codex Babylon, they may have a fighting chance to save humanity from utter destruction.