Holy Wars by Stephan van Hugten.
Long ago, people called Warlocks used Magick to summon spiritual entities. They used these angels or demons to push human civilization to great heights. Unfortunately the demons corrupted the Warlocks and the deranged Warlocks plunged the world into darkness. Eventually an alliance of the last warlocks and the angels saved the world, but Magick was banned forever. Until after many centuries a group tries to relearn this forbidden art. In their foolishness they summon the demonic earl Halphas who immediately seizes this opportunity to conquer the world. Without warlocks, will the angels again be able to stop them?