Disclaimer: I am the Dark Lord to the second power. I control all, including Naruto, and as such am well within my right to publish the truth. All may grovel at my feet.
1.
Naruto is immortal.
He became immortal the moment Yondaime summoned the Shinigami and had the God of Death seal Kyuubi into him.
No, it wasn't because of Kyuubi's chakra, but it certainly helped.
It was actually the Yondaime's deal with the Shinigami – life for death – life for death.
Namikaze Minato's life for Uzumaki Naruto's death.
As a result of this, Minato's soul will never grace the Material Realm ever again; Naruto's soul likewise will never see the Spectral Realm.
2.
Contrary to popular belief, Naruto and Kyuubi's soul are not tied or linked together; instead the Cage that holds Kyuubi prisoner is powered by Naruto's lifeline.
So, if Naruto dies, the Cage fails and Kyuubi becomes free.
But since the Shinigami took away his death, Naruto cannot die and thus the Cage will always be powered and Kyuubi will always be a prisoner.
Theoretically speaking, of course.
3.
Kyuubi didn't become the strongest of the Bijuu and the God of Fox by being a fool. He easily figured out the hole in the Cage the moment he was sealed into Naruto, and could easily escape his Cage. In fact, he does this once to prove that he can do it.
But he doesn't escape the Cage – not even from the moment he was sealed – was just because he could, didn't mean he would.
Naruto begins to hate Minato after that moment, because Minato used him carelessly. He forced Naruto to suffer throughout his life for one purpose, and even the supposedly fool-proof method failed against the all knowing might of the Fox God.
4.
Naruto is Minato's greatest failure. Only Naruto, Minato and Kyuubi know this. Minato will wonder why Kyuubi doesn't escape the Cage, Naruto will wonder how would his life turn out had Minato been successful, and Kyuubi merely laughs in their faces.
5.
Naruto begins to hate Minato even more when he finds out that the God of Death will not take his soul as recompense for summoning him. The Shinigami's reasoning? Since Naruto is the living representative of a god, he is entitled to all immunity of the said god.
Shinigami was actually lying when he said that. He just liked to fuck people over, and Naruto always brought strong people for him to devour. They were tasty.
6.
Kyuubi's chakra actually corrupts and destroys organic materials – the only reason why it reacts the way it does with Naruto's body is because it thinks Naruto is part of Kyuubi.
When Kyuubi was sealed into Naruto, its chakra began to injure Naruto's body. It took three long, pain filled weeks of constant damage for Naruto's body to finally adapt to the foreign chakra.
The constant healing as well as the foreign chakra running in his body was in part responsible for Naruto's abnormally large chakra well and quick healing. The other part was genetic.
7.
Sakura learned about the Kyuubi's chakra properties and its effect on organic materials when a four tailed Naruto attacked her. She strongly suspects that Naruto is immortal, although for the wrong reasons.
8.
Sarutobi Sandaime, on other hand, knows precisely why and how Naruto became immortal.
9.
It does not escape his notice that Orochimaru knew how to become immortal when he defected from Leaf.
10.
The irony is not lost on Sandaime that Orochimaru fled from the Shinigami – arguably the only being who could make him immortal – when he summoned it during the Sand-Sound Invasion.
11.
The irony still isn't lost on the dead Sandaime and living Naruto when Orochimaru realizes the Shinigami's ability and summons him – only to be refused by the God of Death. They both laugh uproariously when Shinigami takes Orochimaru's soul and eat it.
12.
Maito Gai and Rock Lee, as well as the previous practitioners of the Chakra Gates, unknowingly possessed the secret to immortality. The Eighth Gate – the Death Gate – actually releases Death from the practitioner's soul, thus making them immortal. However, those who know this quirk of the Eighth Gate are buried alive – since the practitioners used the Eighth Gate as a last ditch resort and thus damaged their own bodies beyond repair.
Had they bothered to release the Death Gate during peace time and without any strenuous exercise, the users would find themselves immortal. But they did not, and they end up being buried alive.
Subsequently, as a result of this, Maito Gai and Rock Lee (as well as previous practitioners of the Chakra Gates) are buried alive, because there is no need to heal a dead body because it's dead. Right? RIGHT?!
13.
Hidan isn't immortal.
After long hours of mediating, praying, sacrifices and generally practicing Jashin, Hidan is able to have complete harmony between his spiritual and physical bodies. As a result of this, Hidan can remain in the Material Realm through sheer willpower, even when his physical body ceases to work.
He actually has the enviable power of choosing when to die.
14.
Hidan is actually being truthful when he tells the monks and other religious people that they would find spiritual enlightenment under his religion, and subsequently shakes his head at their foolishness when he slaughters them. (Of course, all in the name of Jashin).
15.
The irony isn't lost on Hidan that monks with years of training failed to achieve spiritual harmony, but an extremist cult worshiping the God of Destruction got it right.
16.
Pein, for all of his power, isn't a god, and will never become a god. Naruto proved that by summoning a real god.
Kyuubi liked how Pein tasted.
The Shinigami is miffed.
17.
The real reason why the Death God doesn't take Naruto's soul when Naruto summons him?
It's because Naruto summoned Kyuubi to eat Pein. The Shinigami made Naruto immortal, and as such expects a little loyalty by being summoned before any other gods.
18.
Uzumaki Naruto is the only person in the world who can summon the Death God without giving up his soul.
19.
Uzumaki Naruto is also the only person in the world whose soul the Shinigami will not touch.
20.
Uzumaki Naruto is the only person in the world to become a Jinchuuriki before the Kyuubi was sealed into him. He is in essence, a double Jinchuuriki. Only Sandaime, Naruto, Shinigami and Kyuubi know of this.
--Fin--