In the Forgotten Realms, it is pretty much required.
IMC, it isn't. Basically, it has to do with afterlife.
If you follow one of the gods from the pantheon, then your soul goes to the godworld (the moon).
If you followed one ot the fiendish cults, then your soul is torn away from reality and used as construction material to create a new world for the fiends. Yay!
If you were animistic or faithless, then your soul stays around a little bit and reincarnates in something else. This is what happens to animal souls, for example.
As a result, it's easier to resurrect deists (followers of a god) than animists or atheists, because the latters' souls will, after a short time, be back in a new existence, while the bulk of former are stored within gems in some deity's hoard... (Only truly exceptional individual are allowed to "live" on the Moon, otherwise there would be overpopulation problems.)