There is a weird repetitive thing in fan fictions where Rias Gremory is not a very good person and tricks essay into becoming a devil by letting him die at the hands of Raynare. There is supporting evidence that she gave her summoning circle to her familiar to use allowing her to revive Issei. Although there is no evidence that disproves that she has done this there is still doubt within my mind that she truly went through with it, although with all of the evidence that I have found it is pointing towards her doing it. One justification that I have seen for her is that the interspecies relationships between fallen angels and devils would have led to a war if she had killed off the stray Fallen angels in the church. I however believe that because the territory is owned by devils and that is public knowledge the fallen angels were never allowed to be there in the first place. Because Rias gives her familiar a teleportation sigil we can use that to decipher the fact that she had already known Issei was going on a date with a fallen Angel, even though we know for a fact the fallen angels were not permitted to be in the territory. Another thing we know is that the Shinto pantheon holds domain over Japan and would have not taken a liking to the fact that humans were being killed in their domain. Asia was one of multiple killings done by the church and the fallen angels. Freed Selzen was a rogue exorcist that had killed a bare minimum of three people In the town of Kuoh, let alone anywhere else he may have committed atrocities. Galper Galilei was already running rampant in their territory. Galper almost started the next war when he fused the stolen shards of Excalibur together and then gave them to Freed. Kokabiel was the leader of this whole kill the town of Kuoh scheme if Kokabiel got away with killing Rias and Sona an entire new war would have taken over where the devils may have come out on top. The devils holding two spots from the top 10 would've had a easier time dealing with the Gregory because at this point they only had one or two longinus wielders and their Cadre's, the angels wouldn't have fared any better because they pretty much only had their archangels and Dulio, and the devils had Lucifer Belzebubl and the other two Maus, plus they're Satan class, and their ultimate class devils. Even if the devils came out victorious they would have too little numbers to be of any significant value while the other pantheons tore them asunder. Back on topic, Between season three and season 4 Issei had become friends with a lot of strong people, if what Gremory had done holds true and Issei found out the devils would not only have to be worried about the red dragon emperor, they would have to be worried about what Odin, Azazel, Yasaka, etc... Would do. At this point Azazel sees Issei as more of a surrogate son. Odin has a soft spot for the fellow pervert. Yasaka loves him, enough said. And because Yasaka holds ties with the Shinto pantheon they would have to worry about beings like Amaterasu, Tsukiyomi, and Susanoo and these three are powerful gods in and of themselves. There are more powerful deities that he is friends with, whether he knows it or not. In the light novel Shiva sees him as a little brother, Shiva being ranked second or third on the top ten list would prove detrimental if Issei were to come upon the fact that he was tricked and forced into being a devil. I believe if what had happened to him became public the lower class devils would throw a riot, this would eventually lead to Saji trying to figure out what happened to his rival, Sairorg Would be highly disappointed in his cousin, and Vali, who at this point considers Issei more of a brother, would be absolutely livid. Ophis who sees issei as a... ally? Would have a reaction that I don't think anybody could theorize upon. Rossweisse would be angry and probably sad at learning the past of Issei. Irina would be super saddened but also try and help Issei in the best way she can, by being there for him. Xenovia would be angered at the fact that Issei was tricked into be being a devil, and would take a similar approach to Irina. Ravel would also be one to try to support Issei because she loves him, god damn tsunderes. Kuroka would try to figure out if Koneko had anything to do with Issei's reincarnation, And if she did she would probably be very highly disappointed while also trying to help Issei in any way she can, and I mean in "any" way she can. All in all Issei has very powerful allies that would not turn their back on him if he were to ever find out the truth. I would love to find a piece of fiction where he joins the Yokai faction after learning about the truth of his reincarnation. Azazel Would accept whatever Issei does and would try to help him however Issei asks, most likely with training. Yasaka would take Issei in, in a heartbeat, she loves him and we very much know that in the light novel. She has been shown to be very unsubtle in her advances towards giving Issei affection. Michael and Gabriel also hold soft spots for the powerful pervert. It appears that Dulio holds respect for him and that means he may try to help him when the time comes. Roygun Belphegor wants to become his wife. Le Fay likes him and Arthur respects him, even though Arthur is trying to get him with his sister. Sun Wukong likes the kid he even volunteered to train him and Vali. We do not know much about him and great Reds relationship but if Reds love for the oppai theme song is anything to go by, red wouldn't be too happy with Isaei being hurt.
Thank you for coming to my Ted talk!