User blog comment:Sinister 00/About 666/@comment-16281343-20140103083241/@comment-24230475-20140113210948

key factor to bringing back someone from the dead, if it's someone powerful, you keep the method secret, not the deed. If God comes back it will mean that the same method can be applied to the Four Satans as they are of the same nature. you avoid that by resurrecting God through furtive and secret methods and you make sure no one learns how you did it, knowledge is the ultimate power and when you keep it you become the exclusive wielder of that power. Also a resurrection like that involves great preparation and planning to begin with, it's not that simple and automatic. You probably need to gather components a la Dragonball and perhaps a ritual or two to go with it that requires their own components and energy reserve. As to Issei being the vessel of God it doesn't really matter that he died twice. He may need to die a third time to activate the God essence  in him per the power of three that characterizes holy and arcane lore. the possibility lies that either Rias or Asia is or could be the reincarnation of Mary or both could be half of the whole persona(like Xenosaga). Just tossing ideas here though, I could be wrong on all counts, depends on Ichibumi and how he feels when he starts writing or if he sees this.