Angelology by Danielle Trussoni is set in the last weeks of the Twentieth century and in the years up to and during World War Two. A war between the angels and the nephilim and humanity will be screwed in the process. In the tradition of the religious horror of the 1960s and 1970s, the heros here are religious scholars (angelologists) and nuns. In it's set up, it's most like The Sentinel by Jeffrey Konvitz — except it's about three times as long.
A mysterious and uninvited visitor to a convent in upstate New York, leads Sr. Evangeline onto the discovery of a life time. But she might be too late to save the world. But of course she patiently listens to the elderly Sr. Celestine's long and rambling tales of her life and research in France on the eve of World War Two.
For the reviews that compare Angelology to the Da Vinci Code, I disagree. Although both share Catholic mysticism, they are very different in their narrative structures. Dan Brown's books are basically capers with the good guys either racing to the next clue before the bad guys, or running away from the bad guys. It's charming and silly and there's no real expectation to take any of it very seriously. Angelology is the exact opposite except that it's just as full of plot holes as Brown's books but it takes itself so deadly seriously that there's no fun to be had.
Rather than leaving the plot to a young nun with a closetful of skeletons, what the world needs is the brothers Winchester and their porn watching, booze swigging, on-again off-again angel, Castiel.