Impure: The Pureblood Chronicles

Eric D. Claymore is a Pureblood, a human born and blessed by the Holy Virgin Mary. As the war wages on between sin and purity, Eric faces the dark lustful forces of Virgin Mary's evil twin sister, Black Mary, birth-giver of demons. The two sisters pit their forces against each other, and Eric is given the task of protecting a school that he is assigned to.