We got a tip and dived into it! Religion was once in Civilization V, but later removed by Firaxis. Many files of the game have links to religous elements of the game, including a religious victory, Shrines, State Religions and Missionaries. Firaxis never denied that they considered to bring religion back in Civilization 5, after it was a big success in Civilization IV!
Eight religions are named in the source files: Buddhism, Christianity, Hinduism, Islam, Polytheism, Shintoism, Spiritualism and Zoroastrianism. (Compared to the seven religions of Civ4). Apparently the developers had already proceeded beyond just 'copying and pasting' the religion-concept from Civ4, and even had decided upon the names of the to-be-included religions. Later it appeared that Religion was not compatible with the new diplomatic / attitude model that was being implemented in the game, Jon Shafer (Lead Designer of Civilization V) answered in various interviews. We were not aware yet that the decision was made when Religion was already in a more or less working state in the game at that moment.
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From the files we learn that a Religious Victory would be about bringing your religion to 'all cities'! After completing a shrine the game would tell you: "You have completed a majestic Shrine in CityName to honor ReligionName! You may now train Missionaries there to spread the faith." Other lines found in the language-files are, ao.: "By completing your empire's Shrine, you may now adopt a State Religion!", "An unmet Player has adopted a Religion!"
The unit files show that some units may 'found' a religion. It's unsure if this concerns missionaries that 'found' religions in different cities, though 'spreading' would a better term then. It's more probable that some units would be able to found a religion. Perhaps something like a Great Prophet, though the files show no sign of the former existence of such a unit. That does not mean that such a unit was never there. The files show that all factual religion links to the game have been removed, like units, buildings, etc. The last revelation is the lost religious 'flavor', which is still in the game, though not assigned to any Civilization or City State.
There are rumors in the forums about religion being brought back in a possible future Expansion Pack for Civ5. A poll to bring back Religion is already available. For that to happen religion must find new ways to connect to other parts of the game, since the diplomatic path for religious influence has been blocked by Firaxis for sure.
All religious references in Civilization V:
TXT_KEY_PRODMOD_RELIGION_PLAYER
[NEWLINE][ICON_BULLET]Empire Modifier for Religious Builds: {1_Num}%
TXT_KEY_RELIGION_BUDDHISM
Buddhism
TXT_KEY_RELIGION_CHRISTIANITY
Christianity
TXT_KEY_RELIGION_HINDUISM
Hinduism
TXT_KEY_RELIGION_ISLAM
Islam
TXT_KEY_RELIGION_POLYTHEISM
Polytheism
TXT_KEY_RELIGION_SHINTOISM
Shintoism
TXT_KEY_RELIGION_SPIRITUALISM
Spiritualism
TXT_KEY_RELIGION_ZOROASTRIANISM
Zoroastrianism
CIV5Buildings.xml
FoundsReligion: type="boolean" default="false"
IsReligious: type="boolean" default="false"
CIV5Buildings.xml
CIV5Flavors.xml
CIV5Policies.xml
CIV5Projects.xml
CIV5Victories.xml
Notifications.xml
CIV5GameTextInfos2.xml
CIV5GameTextInfos_WorldView.xml
CIV5Units.xml
It's fun to find this stuff in the source files. For you it's not news, you were most probably there when the ditched it. But for us it's the first time we find out how religion was beings haped before it was ditched.
It's not a surprose, btw, as being said by many people in this thread, and in the article
Why are you downplaying the news, THAT's the real question