Naruto: Hidan's Immortality, Explained

gamerant.com:

Among the Akatsuki's members in Naruto Shippuden are Hidan and Kakuzu, often referred to as the immortal duo. This is a bit of an exaggeration. Kakuzu is extremely difficult to kill not because he's truly immortal but because he can have up to five hearts at any time, and can only be killed after all of them are destroyed. He could replace them by taking hearts from those he killed, gaining access to their Chakra Natures in the process.

While this makes him a frightening opponent, he was eventually killed in battle by Naruto and Kakashi. Hidan, on the other hand, is perhaps the only truly immortal character in the series, unless you count Orochimaru's ability to maintain his consciousness through curse marks.

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The source of Hidan's immortality lies in his fervent worship of the evil god Jashin. Little is known about the religion, to the extent that the Leaf Village once thought Hidan had invented it. Hidan believes that Jashin requires regular sacrifices, and thus is a prolific killer. His immortality is implied to be the result of sacrificing many people at once to Jashin, but the exact mechanism for how he acquired it remains mysterious. Ryuki, another Jashin follower introduced in Boruto, is not immortal, and the plot centers around him trying to gain immortality by sacrificing a large number of young women. This implies that not all followers of Jashin are immortal, and indeed Hidan is the only confirmed case.

The extent of Hidan's immortality is frightening. He has survived multiple stabbings, a massive blast of fire from the Two Tails, decapitation, and having his body blown to pieces by paper bombs. If stabbed or burned, he can regenerate from his injuries