From The Vault

All the basic pre-war belief systems are still active after the Great War. The entire spectrum of Christianity still exists, and has scattered into even more splinter groups. Mormonism still exists, since it was hard to nuke all of Utah, and Mormons are pretty hardy folk. The Hubologists are active even more than before the war. Followers of Dharma also exist.

Of course, many new religions also came into being. Just before the war, and soon after it, many doomsday cults were created, one of which eventually became the Children of the Cathedral. Some of the post-war factions can also be considered quasi-religious even if they do not worship any deity - the Followers of the Apocalypse worship knowledge, while the Brotherhood of Steel worships technology.

There are also as many tribal religions - some worship fragments of forgotten knowledge about the world before the War, some, like Arroyo or Sulik's tribe, worship their ancestors, and many others deify the Nature, or particular plants or animals (like the Vipers).