According to the official character databook:
The Way of Jashin uses bodies of believers in forbidden jutsu rituals. Hidan was the first successful test case.
As a reward for his many massacres, Hidan acquires an immortal body via the Way of Jashin. This only helps to affirm his love for the Way.
Thou shalt kill thy neighbor. This dogma is contradictory since it would allow devotees to kill each other. But to Hidan who is immortal, this is meaningless. That is why he is able to accept the doctrine and follow its teachings. Even if he ends up with just his head...
It also adds, regarding his motivations:
The Way of Jashin is absolute for Hidan, the only thing worth believing in. He considers his supreme purpose to be to spread the Way of Jashin to nonbelievers throughout the world. It means to create a world that acknowledges mass killing. And Hidan feels that this is possible by being a part of the Akatsuki.
And this is about it, regarding to info on Hidan on the databook.