User blog comment:B214/Volume 21 info & spoilers/@comment-83.22.168.173-20160711150248/@comment-9595671-20160821000718

Remember there are multiple gods in the series, so that doesn't mean the Biblical God. Also Issei is a Devil, so I don't think he could draw any power from him. So if there's any God the G refers to, then it would be Ophis in this case.