User blog comment:B214/Question and Answers 6/@comment-201.215.165.7-20161220161455/@comment-9595671-20161220223830

From what I can recall, in DxD, Chrstianity/Judism/Islam and Hindu/Buddhism (and possibly Japanese/Shinto, harder to say because no japanese gods have shown up yet) are mixed together for the sake of the author being able to manage both easier. Since there's a lot of shared things between them, it works out.